Helmer Fjellvåg

• Mohn, Chris Erik; Fjellvåg, Helmer; Ponniah, Vajeeston; Valldor, Björn Martin & Bergum, Kristin (2026). Benchmarking Density Functional Theory for Accurate Calculation of Nitride Band Gaps. Journal of Chemical Theory and Computation. ISSN 1549-9618. 22(3), s. 1321–1337. doi: 10.1021/acs.jctc.5c01703. Fulltekst i vitenarkiv Vis sammendrag

  We benchmark exchange-correlation functionals for the calculation of fundamental band gaps of inorganic nitrides. These include conventional functionals such as the local density approximation (LDA), the generalized-gradient (Perdew–Burke–Ernzerhof) approximation (PBE), simple Slater exchange functionals (SLOC), specialized LDA/GGA-derived high local exchange (HLE16) and Armiento–Kümmel semilocal (AK13) functionals, meta-GGA functionals including TASK, the modified Becke–Johnson functional (mBJ), and Heyd–Scuseria–Ernzerhof (HSE06) hybrid functional, as well as quasiparticle GW theory. Since inorganic nitrides remain strongly under-represented in previous extensive benchmark studies, the current subdatabase contributes towards building a future large-scale balanced materials compilation of band gaps to benchmark theory. From a literature survey, we carefully collect 25 binary and 11 ternary nitrides with a focus on semiconductors spanning the periodic table, including ionic Li3N, antibixbyite-structured X3N2 (X = Be, Mg, Ca), early transition metals and lanthanides (e.g., ScN, YN, and LaN), ultrahard Th3P4-type structured M3N4 (M = Zr, Hf) compounds, promising photocatalysts Ta3N5, different polymorphs of III–V reference covalent nitrides (BN, AlN, GaN), and many M3N4 polymorphs (M = C, Si, and Ge) such as spinel-structured phases. Consistent with previous extensive benchmark tests, conventional LDA/PBE unsystematically largely underestimate band gaps with mean absolute errors (MAE) of >1.0 eV and mean absolute percentage errors (MAPE) of about 50%. Simple Slater exchange functional, SLOC, the GGA-derived AK13LDA and HLE16 functionals show improvement over LDA/PBE with MAE of 0.5–0.6 eV (MAPE ∼ 20–25%) with mBJ and HSE06 being the most accurate, with MAE = 0.30 and 0.28 eV (MAPE 12.1% and 11.1%), respectively. Strategies for the development of machine learning and the choice of appropriate exchange-correlation functionals for high-throughput large-scale material screening are discussed in light of these results.

• Yartys, Volodymyr & Fjellvåg, Helmer (2025). Structural chemistry of metallic hydrides. I Yartys, Volodymyr (Red.), Metal Hydrides for Hydrogen-Based Energy Storage, Volume 1: Fundamentals. Woodhead Publishing Limited. ISSN 9780443333712. s. 265–307. doi: 10.1016/B978-0-443-33370-5.00008-8. Fulltekst i vitenarkiv Vis sammendrag

  Metal hydrides show a close interrelation between their crystal chemistry, H storage capacity and stability, thus allowing optimization of the H storage performance. Typical structural features of the hydrogenation–average volume expansion of 2.5-3 Å3/atom H, non-directional hydrogen-metal bonding, simultaneous partial occupancy of the various types of sites, preferred filling of sites enriched by the large hydride-forming metals having radii exceeding 0.4 Å while the shortest H–H separations are normally longer than 1.9 Å–do not describe the rich crystal chemistry features of the metal-hydrogen interactions occurring in the ternary metal hydrides. Further to the typical and well-known structural features of a large variety of the metal hydrides RxTyXz-H (R = Rare Earth Metal; Zr, Ti, Hf; Ca, Mg; T = Fe,Co, Ni; X = B,Al,Ga,Si,Sn), we will consider various unusual transformations involving a) hydrogen assisted rebuilding of the metal sublattice into a different type of the structure (MgNi2H3); b) formation of directional covalent bonding Ni–H in the Ni-containing ternary hydrides (CeNi3H2.7); c) volume contraction of the Me–H systems on hydrogenation (ScNiSnH0.5); d) chemical influence of magnesium on the crystal chemical features of the formation of interstitial hydrides [La3-xMgxNi9-H2]; e) formation of anomalously short H.H separations [(La/Nd)NiInH1.3]; and f) hydrogen ordering resulting in the formation of superstructures with a full occupancy of the H sites [Zr-based Laves phase hydrides]. We conclude that the stability of the metal hydrides is defined by a balance between the energy released due to the formation of the Me–H bonds and energy consumption required to achieve deformation of the metal sublattice, and that various structural phenomena accompanying the hydrogenation are caused by the modification of a long-range structural order, changes of the magnetic order and the valence state of the constituting metal atoms.

• Hval, Halvor Høen & Fjellvåg, Helmer (2025). Tracking phase transformations in LiMn1.5+xNi0.5−xO4 by high resolution X-ray diffraction. Journal of Materials Chemistry A. ISSN 2050-7488. 13, s. 41122–41143. doi: 10.1039/d5ta05130d. Fulltekst i vitenarkiv Vis sammendrag

  The stability and structural aspects of three categories of quenched and annealed LMNO samples, representing different stages in the processing of realistic materials, were studied by synchrotron powder X-ray diffraction and Raman spectroscopy. The complex Li–Mn–Ni–O energy landscape opens for coexistence of up to five phases under non-equilibrium conditions. These were evaluated by Rietveld refinements, assisted by lattice parameter – composition correlations, which quantitatively confirmed the nominal bulk composition. Mn/Ni cation diffusion occurs at much lower temperatures (400 °C) than previously anticipated, evidenced by Mn/Ni cation ordering in LMNO and by the (back-)formation of rock salt (o-RS) and N-layered impurity phases. Raman data reveals local Mn/Ni ordering well before the formation of domains detectable by X-ray diffraction. The formation of o-RS and Mn/Ni ordering occurs within the same temperature window. We find that o-RS is most likely the cause for oxygen release, without formation of O-vacancies in the parent spinel. A Mn-enriched e-LMNO spinel phase can form at low temperature alongside the ordering process and back-transformation of Ni-rich impurities. Such Mn/Ni segregations are the main cause of Mn(III) formation in LMNO electrodes. The described procedure provides a basis for detailed analysis of practical LMNO materials, emphasizing that the o-RS phase must be considered an integral part of all such analyses. Operando SXRD data further show that ordered and disordered LMNO can be distinguished during cycling, that tetragonal variants form at deep discharge, and that o-RS remains largely undetectable due to its small amount.

• Fjellvåg, Øystein Slagtern; Saha, Dipankar; Chernyshov, Dmitry; Bernal, Fabian Leonardo Martinez; Fjellvåg, Helmer & Hauback, Bjørn Christian [Vis alle 7 forfattere av denne artikkelen] (2025). On the low temperature crystal structure of KCrF3. Journal of Solid State Chemistry. ISSN 0022-4596. 352. doi: 10.1016/j.jssc.2025.125558. Fulltekst i vitenarkiv Vis sammendrag

  We report on the low-temperature phase transition and crystal structure of KCrF3, revisiting the previously reported reentrant tetragonal transition at 5 K. Using high-resolution X-ray powder diffraction across a temperature range of 5–300 K in two independent experiments, we study the structural developments and apply symmetry-mode analysis to describe the transition from tetragonal to monoclinic. A tilt of the CrF6-octahedra drives the symmetry reduction. Evaluation of the bond lengths and angles shows no sign of a reentrant or incipient transition. Our findings confirm that the low-temperature phase of KCrF3 is monoclinic.

• Fjellvåg, Øystein Slagtern; Park, Heesoo; Veillon, Fabien; Lyu, Jike; Medarde, Marisa & Amedi, Salah Bra [Vis alle 10 forfattere av denne artikkelen] (2025). ACrF3 Jahn–Teller-Distorted Fluoroperovskites: Expanding to RbCrF3 and CsCrF3. Inorganic Chemistry. ISSN 0020-1669. 64(36), s. 18316–18322. doi: 10.1021/acs.inorgchem.5c02668. Fulltekst i vitenarkiv Vis sammendrag

  The Jahn–Teller effect significantly affects the CrF6 octahedra in Cr(II) fluoroperovskites. Here, we report the synthesis, crystal structures, and magnetic properties of RbCrF3 and CsCrF3, thereby completing a comprehensive investigation of the ACrF3 fluoroperovskites. Powder samples are prepared using a wet-chemical method, which allows stabilization of Cr(II). RbCrF3 predominantly adopts the P4/mbm space group; however, the absence of the expected (211) peak suggests a limited correlation length, and we demonstrate that this is a result of stacking faults in the rotation of the distorted CrF6 octahedra along the c-axis. We estimate around 5–10% of I4/mcm-type stacking in P4/mbm for RbCrF3. In contrast, CsCrF3 showcases a two-phase coexistence of I4/mcm and P4/mbm polymorphs and, as such, parallels to KCuF3. We discuss and highlight the interplay between the impact of A-site cations and the Jahn–Teller effect acting on CrF6 octahedra. Antiferromagnetic interactions dominate the paramagnetic regime, and RbCrF3 and CsCrF3 both have a Weiss temperature of −12 K, associated with Néel temperatures of 50 and 47 K, respectively. The effective paramagnetic moments of 4.65 and 4.68 μB for RbCrF3 and CsCrF3 are in good agreement with S = 2 spin configuration.

• Fjellvåg, Øystein Slagtern; Fjellvåg, Helmer; Hessevik, Julie; Sjåstad, Anja Olafsen & Steciuk, Gwladys (2025). Crystal Structure of Nd10.67Pt4O24, a New Neodymium Platinate. ACS Omega. 10(12), s. 12487–12494. doi: 10.1021/acsomega.5c00031. Fulltekst i vitenarkiv Vis sammendrag

  A new platinate was recently discovered when Nd2O3 was explored as a platinum capture material in the Ostwald process, formed by a direct reaction between gaseous PtO2 and Nd2O3. The crystal structure of this new platinate and its composition, Nd10.67Pt4O24, are here reported for the first time. The compound is synthesized either by a direct reaction between PtO2(g) and Nd2O3 or by the citric acid chemical route. Based on 3-dimensional electron diffraction data and Rietveld refinement of high-resolution synchrotron and neutron powder diffraction data, we describe its crystal structure in space group I41/a. The compound is structurally related to the Ln11–xSrxIr4O24 (Ln = La, Pr, Nd, and Sm) phases with a double perovskite (A2BB’O6)-like crystal structure with A-site cation deficiency. Owing to the fixed oxidation state of Pt(IV), two of the four Nd sites are partly occupied to provide charge neutrality, with Nd4 taking a split position. On heating, Nd10.67Pt4O24 decomposes into Nd2O3 and Pt. A plateau in the thermogravimetric curves measured in 33 vol % O2 in N2 indicates the presence of an intermediate Pt(II) phase at around 960 °C, probably isostructural with La4PtO7.

• Gonano, Bruno; Fjellvåg, Øystein Slagtern; Steciuk, Gwladys; Marshall, Kenneth; Fjellvåg, Helmer & Valldor, Björn Martin (2025). PVDF fluorination of La₄Co₃O₁₀: Synthesis, structure and physical properties of La₄Co₃O₁₀F₂. Scripta Materialia. ISSN 1359-6462. 255. doi: 10.1016/j.scriptamat.2024.116379. Fulltekst i vitenarkiv Vis sammendrag

  Topochemical fluorination of oxides recently gained interest as it opens the way to substantial structural modifications and drastic changes in physical properties. We present a study of La4Co3O10F2, synthesized via low-temperature topochemical methods using polyvinylidene difluoride (PVDF). Using synchrotron X-ray powder diffraction, we find the compound to crystallize in a monoclinic cell (A2/a, with a = 5.32969(9) Å, b = 5.37555(1) Å, c = 30.5958(6) Å and β = 90.994(1) °), where the fluoride anions occupy the rock-salt layer's interstitial sites, inducing a tilting of the CoO6 octahedra. The physical properties of this novel compound are compared with those of the parent compound La4Co3O10, evidencing changes in resistivity and magnetism, highlighting the possibility of tuning magnetic interactions and electronic correlations through PVDF fluorination.

• Skurtveit, Amalie; Jr, Andrew Pastusic; Brennhagen, Anders; Maddar, Faduma M.; Mohn, Chris Erik & Karmakar, Abhoy [Vis alle 13 forfattere av denne artikkelen] (2025). Unique Na5−xSbSe Phase Enables High-Rate Performance of Sb2Se3 Anodes in Na-Ion Batteries. Advanced Energy Materials. ISSN 1614-6832. 15(31), s. 1–16. doi: 10.1002/aenm.202501433. Fulltekst i vitenarkiv Vis sammendrag

  Na-ion batteries (NIBs) need new anode materials to improve energy density. Metal chalcogenides, such as Sb2Se3, represent a promising alternative to commonly used hard carbon materials, demonstrating high-rate performance up to 5 A g−1 with minimal capacity losses. However, Sb2Se3 is believed to operate under the conversion/alloying mechanism, typically linked with large structural transformations and volumetric changes—quite contrary to its performance. Herein, by combining multiple operando techniques and atomistic simulations, a new fully sodiated phase, Na5−xSbSe, is unambiguously revealed as the origin of the high-rate performance of Sb2Se3. Na5−xSbSe is stable within 0.01–0.80 V versus Na/Na+ and crystallizes in I4/mmm. The remarkable structural flexibility of Na5SbSe to changes in Na-content allows the anode to be (de)sodiated with minimal volumetric changes (≈3.4%). This unique “breathing effect” is intimately linked to high inherent vacancy concentration, disordered, and structurally flexible anion sublattice, providing a stable framework for fast Na diffusion, contributing to the fast-charging properties of Sb2Se3. The study showcases the power of operando methods for discovering new phases that are hidden in the mechanistic paths of well-studied reactions and underlines the intertwined nature of various characterization methods assisted by atomistic insights for a comprehensive understanding of complex (de)sodiation mechanisms.

• Hessevik, Julie; Carlsen, Cathinka Scheele; Bestul, Oskar K.; Waller, David; Fjellvåg, Helmer & Sjåstad, Anja Olafsen (2025). Oxides for Pt Capture in the Ammonia Oxidation Process—A Screening Study. Reactions. 6(1). doi: 10.3390/reactions6010013. Fulltekst i vitenarkiv Vis sammendrag

  Metallic Pd/Ni gauzes, located downstream of the Pt/Rh ammonia oxidation catalyst nets in the Ostwald process, is the current technology for capturing volatile gas phase platinum and rhodium species lost from the Pt/Rh combustion catalyst through evaporation. In this screening study, we explore four oxide families, ABO3 perovskites, (ABO3)n(AO) Ruddlesden–Popper (RP) phases, AO rock salt, and A2O3 sesquioxide type oxides, as alternative materials for platinum capture. It was found that all the tested nickelates, LaNiO3, NdNiO3, La2NiO4, and La4Ni3O10, captured platinum well and formed A2NiPtO6. In contrast, La0.85Sr0.15FeO3, LaFeO3, and LaCoO3 did not capture platinum. CaO, SrO, and Nd2O3 formed low-dimensional platinates such as CaxPt3O4, Sr4PtO6, and a newly discovered neodymium platinate, Nd10.67Pt4O24. Gd2O3 did not capture platinum in bench-scale experiments in dry air, but did, however, seem to capture platinum under pilot plant conditions, likely due to the co-capture of Co lost from the N2O abatement catalyst. The catalytic activity of both oxides and platinum-containing products were studied, toward NOx and N2O decomposition. None of the oxides showed significant activity toward NOx decomposition, and all showed activity toward N2O decomposition, but to different extents. An overall assessment of the screened oxides with respect to potential use in industrial Ostwald conditions is provided. All tested oxides except CaO and SrO withstood industrial conditions. From our assessments, the nickelates and A2O3 (A = Nd, Gd) stand out as superior oxides for platinum capture.

• Hessevik, Julie; Sønsteby, Henrik Hovde; Fjellvåg, Helmer & Sjåstad, Anja Olafsen (2025). Reactivity of gaseous PtO2 and RhO2 with LaNiO3 thin films: a systematic XPS study. Materials Advances. 6(3), s. 1097–1106. doi: 10.1039/D4MA00974F. Fulltekst i vitenarkiv Vis sammendrag

  The reaction between volatile platinum and rhodium gas species and bulk materials (alloys or oxides) is of high practical importance in the chemical process industry. Herein, an X-ray photoelectron spectroscopy (XPS) study has been conducted to understand how PtO2(g) and RhO2(g) react with LaNiO3 (LNO) thin films grown via atomic layer deposition (ALD) at elevated temperatures (900 °C). In this study, XPS data for reference powders of La2NiPtO6, LaNi0.95Pt0.05O3, LaNi0.88Rh0.12O3 and LaRhO3 provide a library for the interpretation of oxidation states of platinum and rhodium after reaction with LNO thin films. Upon short exposure to PtO2(g) and RhO2(g), platinum appear as Pt(III) in the LNO surface, but after prolonged exposure, Pt(IV) appears. Complementary diffraction analysis shows that LaNi1−xMxO3 solid solution forms initially where Ni, Pt and Rh are predominantly +III, however, quickly accompanied by the formation of a La2Ni2−2xM2xO6 (M = Pt, Rh) double perovskite phase with its characteristic (004) peak. Rhodium appears as Rh(III) and Rh(IV) in both the R[3 with combining macron]c and the double perovskite like phases. Furthermore, platinum and rhodium depth profiles were generated by the combination of XPS and Ar+ sputtering. Platinum depth profiles reveal major differences between as-grown ALD films and post-annealed films when exposed to PtO2(g). Pt penetrates faster into the LNO material for films without post annealing, probably due to its smaller grain size and larger surface area and grain boundaries. When exposed to a mixture of PtO2(g) and RhO2(g), the Rh : Pt ratio in LNO increases upon prolonged reaction time. These results may have implications on the use of LNO as platinum and rhodium capture material in the Ostwald process.

• Fjellvåg, Øystein Slagtern & Fjellvåg, Helmer (2024). Sn(SO4)2·2H2O from synchrotron powder data. IUCrData. 9(12). doi: 10.1107/S2414314624011799. Fulltekst i vitenarkiv Vis sammendrag

  Tin(IV) sulfate dihydrate, Sn(SO4)2·2H2O, was prepared in a reflux of sulfuric acid under oxidizing conditions. Its crystal structure was determined from powder synchrotron X-ray diffraction data and is constructed of (100) layers of [SnO4(H2O)2] octa­hedra (point group symmetry 1) corner-connected by sulfate tetra­hedra. Hydrogen bonds of moderate strength between the water mol­ecules and sulfate O atoms hold the layers together.

• Hempel, Frida Sveen; Sławiński, Wojciech A.; Arstad, Bjørnar & Fjellvåg, Helmer (2024). Superstructure of Locally Disordered Na2Zn2TeO6. Chemistry of Materials. ISSN 0897-4756. 36(22), s. 11084–1109. doi: 10.1021/acs.chemmater.4c01953. Fulltekst i vitenarkiv Vis sammendrag

  This work expands on the superstructure in the layered oxide Na2Zn2TeO6 (NZTO). The diffractogram contains weak peaks that are not fitted by the average structure but are found to belong to a 2 × 2 × 3 commensurate modulation of the structure. The modulated structure was found by randomizing the starting points to ensure that the global minima are identified. The resulting superstructure is then used to generate a starting structure for reversed Monte Carlo (RMC) fitting of the total scattering data. We identify two structural features that are preserved through both fittings. One is that two of the Zn/Te columns in the layer direction have a significantly higher degree of disorder, with the exchange of Zn and Te. The second feature is that one of the crystallographic Na sites (2a) was found to be empty. The Na atoms are located midway between crystallographic 2a and 6g sites, which surrounds half of the 2a sites. This type of off-centering is found both in the modulated structure and in the corresponding structure preserved through RMC modeling. The results are discussed in relation to variable-temperature solid-state NMR data measured between 100 and 293 K. We suggest that the phenomenon is a feature of a dynamic situation and an important clue to progress the understanding of ion dynamics in more detail.

• Brennhagen, Anders; Skautvedt, Casper; Cavallo, Carmen; Wragg, David Stephen; Koposov, Alexey & Sjåstad, Anja Olafsen [Vis alle 7 forfattere av denne artikkelen] (2024). Unraveling the (De)sodiation Mechanisms of BiFeO<inf>3</inf> at a High Rate with Operando XRD. ACS Applied Materials & Interfaces. ISSN 1944-8244. 16(10), s. 12428–12436. doi: 10.1021/acsami.3c17296. Fulltekst i vitenarkiv Vis sammendrag

  Development of new anode materials for Na-ion batteries strongly depends on a detailed understanding of their cycling mechanism. Due to instrumental limitations, the majority of mechanistic studies focus on operando materials’ characterization at low cycling rates. In this work, we evaluate and compare the (de)sodiation mechanisms of BiFeO3 in Na-ion batteries at different current densities using operando X-ray diffraction (XRD) and ex situ X-ray absorption spectroscopy (XAS). BiFeO3 is a conversion-alloying anode material with a high initial sodiation capacity of ∼600 mAh g–1, when cycled at 0.1 A g–1. It does not change its performance or cycling mechanism, except for minor losses in capacity, when the current density is increased to 1 A g–1. In addition, operando XRD characterization carried out over multiple cycles shows that the Bi ⇋ NaBi (de)alloying reaction and the oxidation of Bi at the interface with the Na–Fe–O matrix are detrimental for cycling stability. The isolated NaBi ⇋ Na3Bi reaction is less damaging to the cycling stability of the material.

• Thøgersen, Rasmus Vester; Hval, Halvor Høen & Fjellvåg, Helmer (2024). Elucidation of the Reaction Mechanisms in Antifluorite-Type Li<inf>5+x</inf>Fe<inf>1-x</inf>Co<inf>x</inf>O<inf>4</inf> Positive Electrodes for Li-Ion Batteries. Batteries & Supercaps. doi: 10.1002/batt.202400348. Fulltekst i vitenarkiv Vis sammendrag

  The Li-rich antifluorite-type oxides Li5FeO4, Li5.5Fe0.5Co0.5O4 and Li6CoO4 have been investigated as positive electrode materials for Li-ion batteries in a combined operando XANES and XRD experiment. All materials show a similar two-step behaviour upon initial charge (termed Stage I and Stage II), and reversibility of subsequent cycling depends upon whether the initial charge cycle is terminated following Stage I or allowed to proceed through Stage II. By tracking the energetic evolution of the XANES pre-edge feature present in both Fe and Co K-edge spectra, as well as the evolution of X-ray diffractograms during charge and discharge, we correlate the changes in chemical coordination and oxidation states in both species and the structural changes to the electrochemical potential profile, and infer the role of anionic redox processes.

• Fjellvåg, Øystein Slagtern; Gonano, Bruno; Bernal, Fabian Leonardo Martinez; Amedi, Salah Bra; Lyu, Jike & Pomjakushin, Vladimir [Vis alle 12 forfattere av denne artikkelen] (2024). Order-to-Disorder Transition and Hydrogen Bonding in the Jahn-Teller Active NH4CrF3 Fluoroperovskite. Inorganic Chemistry. ISSN 0020-1669. 63(23), s. 10594–10602. doi: 10.1021/acs.inorgchem.4c00931. Fulltekst i vitenarkiv Vis sammendrag

  Large quantities of high-purity NH4CrF3 have been synthesized using a wet-chemical method, and its structural chemistry and magnetic properties are investigated in detail for the first time. NH4CrF3 is a tetragonal fluoroperovskite that displays an ordering of the ammonium (NH4+) groups at room temperature and C-type orbital ordering. The ammonium groups order and display distinct signs of hydrogen bonds to nearby fluoride anions by buckling the Cr–F–Cr angle away from 180°. The ammonium ordering remains up to 405 K, much higher than in other ammonium fluoroperovskites, indicating a correlation between the flexibility of the Jahn–Teller ion, the hydrogen bond formation, and the ammonium ordering. At 405 K, an order-to-disorder transition occurs, where the ammonium groups disorder, corresponding to a transition to higher symmetry. This is accompanied by a contraction of the unit cell from breaking hydrogen bonds, similar to the phenomenon observed in water ice melting. The compound orders antiferromagnetically with a Neél temperature of 60 K, an effective paramagnetic moment of 4.3 μB, and a Weiss temperature of −33 K. An A-type antiferromagnetic structure is identified by neutron diffraction, with an ordered moment of 3.72(2) μB.

• Brennhagen, Anders; Skurtveit, Amalie; Wragg, David Stephen; Cavallo, Carmen; Sjåstad, Anja Olafsen & Koposov, Alexey [Vis alle 7 forfattere av denne artikkelen] (2024). (De)sodiation Mechanism of Bi2MoO6 in Na-Ion Batteries Probed by Quasi-Simultaneous Operando PDF and XAS. Chemistry of Materials. ISSN 0897-4756. 36(15), s. 7514–7524. doi: 10.1021/acs.chemmater.4c01503. Fulltekst i vitenarkiv Vis sammendrag

  Operando characterization can reveal degradation processes in battery materials and are essential for the development of battery chemistries. This study reports the first use of quasi-simultaneous operando pair distribution function (PDF) and X-ray absorption spectroscopy (XAS) of a battery cell, providing a detailed, atomic-level understanding of the cycling mechanism of Bi2MoO6 as an anode material for Na-ion batteries. This material cycles via a combined conversion-alloying reaction, where electrochemically active, nanocrystalline NaxBi particles embedded in an amorphous Na–Mo–O matrix are formed during the first sodiation. The combination of operando PDF and XAS revealed that Bi obtains a positive oxidation state at the end of desodiation, due to formation of Bi–O bonds at the interface between the Bi particles and the Na–Mo–O matrix. In addition, XAS confirmed that Mo has an average oxidation state of +6 throughout the (de)sodiation process and, thus, does not contribute to the capacity. However, the local environment of Mo6+ changes from tetrahedral coordination in the desodiated state to distorted octahedral in the sodiated state. These structural changes are linked to the poor cycling stability of Bi2MoO6, as flexibility of this matrix allows movement and coalescence of the NaxBi particles, which is detrimental to the electrochemical stability.

• Saha, Dipankar; Sławiński, Wojciech A.; Kumar, Susmit & Fjellvåg, Helmer (2024). Local Symmetry Deviation from the Average Structure of MnAs Revealed by Pair Distribution Function. Inorganic Chemistry. ISSN 0020-1669. 63, s. 15503–15509. doi: 10.1021/acs.inorgchem.4c02667. Fulltekst i vitenarkiv Vis sammendrag

  MnAs is an interesting material due to its magnetocaloric properties, which can be utilized in magnetic refrigeration. However, despite major efforts, its magnetic refrigeration performances in the substituted forms could not be improved compared to the parent MnAs phase. Both small and big box modeling of the pair distribution function of MnAs for the local structure description and powder X-ray diffraction for the average structure reveal an inherent local orthorhombic distortion in the hexagonal structure of MnAs. As a result of this distortion, any modification to the hexagonal structure results in an orthorhombic structure and a weaker magnetocaloric performance. This study highlights the importance of studying local distortion in magnetic materials. This is achieved by combining X-ray absorption spectroscopy with total scattering X-ray diffraction.

• Hempel, Frida Sveen; Martineau-Corcos, Charlotte; Bianchini, Federico; Fjellvåg, Helmer & Arstad, Bjørnar (2023). Dynamics of Interlayer Na-Ions in Ga-Substituted Na2Zn2TeO6 (NZTO) Studied by Variable-Temperature Solid-State 23Na NMR Spectroscopy and DFT Modeling. ACS Physical Chemistry Au. 3(4), s. 320–405. doi: 10.1021/acsphyschemau.3c00012. Fulltekst i vitenarkiv Vis sammendrag

  Local Na-coordination and dynamics of Na2–xZn2–xGaxTeO6; x = 0.00 (NZTO), 0.05, 0.10, 0.15, 0.20, were studied by variable-temperature, 23Na NMR methods and DFT AIMD simulations. Structure and dynamics were probed by NMR in the temperature ranges of 100–293 K in a magnetic field of 18.8 T and from 293 up to 500 K in a magnetic field of 11.7 T. Line shapes and T1 relaxation constants were analyzed. At 100 K, the otherwise dynamic Na-ions are frozen out on the NMR time scale, and a local structure characterization was performed for Na-ions at three interlayer sites. On increasing the temperature, complex peak shape coalescences occurred, and at 293 K, the Na NMR spectra showed some averaging due to Na-ion dynamics. A further increase to 500 K did not reveal any new peak shape variations until the highest temperatures, where an apparent peak splitting was observed, similar to what was observed in the 18.8 T experiments at lower temperatures. A three-site exchange model coupled with reduced quadrupolar couplings due to dynamics appear to explain these peak shape observations. The Ga substitution increases the Na-jumping rate, as proved by relaxation measurements and by a decrease in temperature for peak coalescence. The estimated activation energy for Na dynamics in the NZTO sample, from relaxation measurements, corresponds well to results from DFT AIMD simulations. Upon Ga substitution, measured activation energies are reduced, which is supported, in part, by DFT calculations. Addressing the correlated motion of Na-ions appears important for solid-state ion conductors since benefits can be gained from the decrease in activation energy upon Ga substitution, for example.

• S, Lathifa Banu; Veerapandy, Vasu; Fjellvåg, Helmer & Ponniah, Vajeeston (2023). First-Principles Insights into the Relative Stability and Physical and Chemical Properties of MoSe2. ACS Omega. 8(15), s. 13799–13812. doi: 10.1021/acsomega.2c08217. Fulltekst i vitenarkiv Vis sammendrag

  A fascinating transition-metal dichalcogenide (TMDC) compound, MoSe2, has attracted a lot of interest in electrochemical, photocatalytic, and optoelectronic systems. However, detailed studies on the structural stability of the various MoSe2 polymorphs are still lacking. For the first time, the relative stability of 11 different MoSe2 polymorphs (1H, 2H, 3Ha, 3Hb, 2T, 4T, 2R1, 1T1, 1T2, 3T, and 2R2) is proposed, and a detailed analysis of these polymorphs is carried out by employing the first-principles calculations based on density functional theory (DFT). We computed the physical properties of the polymorphs such as band structure, phonon, and elastic constants to examine the viability for real-world applications. The electronic properties of the involved polymorphs were calculated by employing the hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE06). The energy band gap of the polymorphs (1H, 2H, 3Ha, 3Hb, 2T, 4T, and 2R1) is in the range of 1.6–1.8 eV, coinciding with the experimental value for the polymorph 2H. The covalent bonding nature of MoSe2 is analyzed from the charge density, charge transfer, and electron localization function. Among the 11 polymorphs, 1H, 2H, 2T, and 3Hb polymorphs are predicted as stable polymorphs based on the calculation of the mechanical and dynamical properties. Even though the 4T and 3Ha polymorphs’ phonons are stable, they are mechanically unstable; hence, they are considered to be under a metastable condition. Additionally, we computed the direction-dependent elastic moduli and isotropic factors for both mechanically and dynamically stable polymorphs. Stable polymorphs are analyzed spectroscopically using IR and Raman spectra. The thermal stability of the polymorphs is also studied.

• Thøgersen, Rasmus Vester; Fjellvåg, Asbjørn Slagtern; Fjellvåg, Øystein Slagtern & Fjellvåg, Helmer (2023). Structural and magnetic properties of antifluorite-type Li5+xFe1−xCoxO4. Zeitschrift für Anorganische und Allgemeines Chemie. ISSN 0044-2313. 649(22). doi: 10.1002/zaac.202300146. Fulltekst i vitenarkiv Vis sammendrag

  Abstract We report on the structural and magnetic properties of Li 5+ x Fe 1− x Co x O 4 (0≤ x ≤1).&lt; The system assumes two distinct crystal structures at room temperature (α and β), and shows a transition in preference from α to β between x =0.18 and x =0.25. This tendency is supported by DFT‐calculations. Upon heating, Li 5 FeO 4 undergoes a reconstructive transition from α→β at ∼1060 K, while Li 6 CoO 4 shows an order‐disorder transition from β→ϵ at ∼885 K, where ϵ is a cubic cation‐disordered antifluorite structure. Li 5.50 Fe 0.50 Co 0.50 O 4 is stabilised towards the ordered β‐phase until at least 1170 K. XANES‐measurements confirm that the oxidation states of Fe and Co remain unchanged throughout the series with charge compensation from additional Li + on increasing x . Magnetic measurements show onset of low‐temperature antiferromagnetic ordering around 3 K to 12 K. The magnetic structure of Li 5 FeO 4 is described on the basis of powder neutron diffraction at 1.5 K, while the magnetic structures of Li 5.50 Fe 0.50 Co 0.50 O 4 and Li 6 CoO 4 are found to be slightly incommensurate below the ordering temperature.

• Choudhary, Mukesh K.; Fjellvåg, Helmer & Ponniah, Ravindran (2023). First principle studies on electronic and thermoelectric properties of Fe2TiSn based multinary Heusler alloys. Computational Materials Science. ISSN 0927-0256. 216. doi: 10.1016/j.commatsci.2022.111856. Fulltekst i vitenarkiv
• Hessevik, Julie; Fjellvåg, Asbjørn Slagtern; Iveland, Oskar; By, Thomas; Skjelstad, J. & Waller, David [Vis alle 8 forfattere av denne artikkelen] (2022). LaNiO3 as a Pt catchment material in the ammonia oxidation process. Materials Today Communications. 33. doi: 10.1016/j.mtcomm.2022.104084. Fulltekst i vitenarkiv
• Thøgersen, Rasmus Vester; Bianchini, Federico; Fjellvåg, Helmer & Ponniah, Vajeeston (2022). Antifluorite-type Na<inf>5</inf>FeO<inf>4</inf> as a low-cost, environment-friendly cathode with combined cationic/anionic redox activity for sodium ion batteries: a first-principles investigation. RSC Advances. 12(27), s. 17410–17421. doi: 10.1039/d2ra01834a. Fulltekst i vitenarkiv Vis sammendrag

  The rapid electrification of our society and the transition towards a larger share of intermittent renewable energy sources in our electricity grids will dramatically increase the demand for cheap energy storage. Sodium ion batteries (SIBs) show a lot of promise to provide the required stationary storage at the grid level at low cost owing to the natural abundance and geographical availability of sodium. In addition, alkali-rich cathode materials exhibiting anionic redox contributions have garnered much attention over the past decade as a strategy to increase the specific capacity. In this work, we investigate for the first time the sodium-rich compound Na5FeO4 as a potential low-cost, environment-friendly cathode for sodium ion batteries from first principles using density functional theory. We investigate three low-energy polymorphs related to the antifluorite structure, verify their dynamical and mechanical stabilities, and show that they exhibit promising ion diffusive properties. As alkali-rich cathode materials are prone to oxygen loss during cycling, we investigate cycling stability with respect to phase transformations and oxygen loss and identify in particular one promising cycling interval that can reversibly shuttle 1.5 Na+ per formula unit between Na5FeO4 and Na3.5FeO4 with a gravimetric energy density exceeding 360 W h kg−1. Investigations into possible redox mechanisms reveal that the charge compensation occurs simultaneously on Fe- and O-atoms in FeO4-tetrahedra, which suggests that Na5FeO4, if realised experimentally as a cathode material, would join the family of combined cationic/anionic redox compounds.

• Hempel, Frida Sveen; Bianchini, Federico; Arstad, Bjørnar & Fjellvåg, Helmer (2022). Effects of Ga Substitution on the Local Structure of Na2Zn2TeO6. Inorganic Chemistry. ISSN 0020-1669. 61(33), s. 13067–13076. doi: 10.1021/acs.inorgchem.2c01431. Fulltekst i vitenarkiv Vis sammendrag

  In the work presented here, we prepared Ga-substituted NZTO (Na2–xZn2–xGaxTeO6, x = 0.00, 0.05, 0.10, 0.15, 0.20) layered materials with a soft chemical, citric acid-based synthesis method and characterized these by means of X-ray diffraction (XRD), 23Na and 125Te NMR, and by density functional theory (DFT) modeling. The influence of randomly distributed Ga cations on the 125Te NMR spectra confirms the successful synthesis. With DFT-based linear response computations, we show that the local distribution of Na ions in the two neighboring interlayers influences the 125Te chemical shift, consistent with observations. DFT modeling suggests that some of the Na sites are rarely occupied in pure NZTO but become favorable upon Ga substitution. There are clear indications that Ga substitution gives an uneven distribution of Na ions in neighboring interlayers and that the Na structure in one layer affects the adjacent layers.

• Eggert, Bruno G. F.; Delczeg-Czirjak, Erna K.; Maccari, Fernando; Kumar, Susmit; Gutfleisch, Oliver & Fjellvåg, Helmer [Vis alle 8 forfattere av denne artikkelen] (2022). Exploring V-Fe-Co-Ni-Al and V-Fe-Co-Ni-Cu high entropy alloys for magnetocaloric applications. Journal of Alloys and Compounds. ISSN 0925-8388. 921. doi: 10.1016/j.jallcom.2022.166040. Fulltekst i vitenarkiv Vis sammendrag

  A series of V1−x-Fe-Co-Ni-Al1+x and V1−x-Fe-Co-Ni-Cu1+x high entropy alloys with varying compositions (0 ≤ x ≤ 0.75) has been investigated for magnetocaloric applications. Compositions were selected according to established properties, such as configurational entropy, atomic size difference, and enthalpy of mixing. To study the influence of composition on magnetic ordering temperatures, the V and (Al/Cu) contents were changed while the content of Fe, Co and Ni was retained at 20 at. % each. The crystal structure and microstructure of the as-cast alloys were compared to literature phase guidelines and thermodynamic calculations based on the CALPHAD approach. The V-Fe-Co-Ni-Al compounds are monophasic and crystallize in a disordered body centered cubic structure or its ordered B2 variant, while the V-Fe-Co-Ni-Cu compounds are all multiphasic. Magnetic transitions in the V-Fe-Co-Ni-Al system span over 400 K, with Curie temperature ranging from 155 K in equiatomic VFeCoNiAl, to 456 K in non-equiatomic V0.25FeCoNiAl1.75. The V-Fe-Co-Ni-Cu alloys display magnetic transitions that span about 150 K, with Curie temperature ranging from 230 K for equiatomic VFeCoNiCu to 736 K for non-equiatomic V0.25FeCoNiCu1.75. The magnetic properties of the V-Fe-Co-Ni-Cu compounds were evaluated by means of density functional theory. Individual element-specific moments, magnetic exchange integrals between atomic pairs, and Curie temperatures were calculated. V0.85FeCoNiCu1.15 is selected due to its Curie temperature of 329 K, and its calculated isothermal entropy change of 0.75 J/kg.K for a field change of 5 T is comparable to other 3d metal-based high entropy alloys that form disordered solid solutions.

• Sottmann, Jonas; Ruud, Amund; Fjellvåg, Øystein; Vaughan, Gavin B. M.; Michel, Marco Di & Fjellvåg, Helmer [Vis alle 9 forfattere av denne artikkelen] (2022). 5D total scattering computed tomography reveals the full reaction mechanism of a bismuth vanadate lithium ion battery anode. Physical Chemistry, Chemical Physics - PCCP. ISSN 1463-9076. 24(44), s. 27075–27085. doi: 10.1039/d2cp03892g. Fulltekst i vitenarkiv Vis sammendrag

  We have used operando 5D synchrotron total scattering computed tomography (TSCT) to understand the cycling and possible long term deactivation mechanisms of the lithium-ion battery anode bismuth vanadate. This anode material functions via a combined conversion/alloying mechanism in which nanocrystals of lithium–bismuth alloy are protected by an amorphous matrix of lithium vanadate. This composite is formed in situ during the first lithiation of the anode. The operando TSCT data were analyzed and mapped using both pair distribution function and Rietveld methods. We can follow the lithium–bismuth alloying reaction at all stages, gaining real structural insight including variations in nanoparticle sizes, lattice parameters and bond lengths, even when the material is completely amorphous. We also observe for the first time structural changes related to the cycling of lithium ions in the lithium vanadate matrix, which displays no interactions beyond the first shell of V–O bonds. The first 3D operando mapping of the distribution of different materials in an amorphous anode reveals a decline in coverage caused by either agglomeration or partial dissolution of the active material, hinting at the mechanism of long term deactivation. The observations from the operando experiment are backed up by post mortem transmission electron microscope (TEM) studies and theoretical calculations to provide a complete picture of an exceptionally complex cycling mechanism across a range of length scales.

• Balakrishnan, Kanimozhi; Veerapandy, Vasu; Fjellvåg, Helmer & Ponniah, Vajeeston (2022). First-Principles Exploration into the Physical and Chemical Properties of Certain Newly Identified SnO<inf>2</inf>Polymorphs. ACS Omega. 7(12), s. 10382–10393. doi: 10.1021/acsomega.1c07063. Fulltekst i vitenarkiv
• Premkumar, T.; Vidya, R. & Fjellvåg, Helmer (2022). Stabilizing p-type conductivity in CuYO2 by co-doping: A first-principles study. Materials Chemistry and Physics. ISSN 0254-0584. 285. doi: 10.1016/j.matchemphys.2022.126101. Fulltekst i vitenarkiv
• Brennhagen, Anders; Cavallo, Carmen; Wragg, David Stephen; Ponniah, Vajeeston; Sjåstad, Anja Olafsen & Koposov, Alexey [Vis alle 7 forfattere av denne artikkelen] (2022). Operando XRD studies on Bi2MoO6 as anode material for Na-ion batteries. Nanotechnology. ISSN 0957-4484. 33(18). doi: 10.1088/1361-6528/ac4eb5. Fulltekst i vitenarkiv Vis sammendrag

  Based on the same rocking-chair principle as rechargeable Li-ion batteries, Na-ion batteries are promising solutions for energy storage benefiting from low-cost materials comprised of abundant elements. However, despite the mechanistic similarities, Na-ion batteries require a different set of active materials than Li-ion batteries. Bismuth molybdate (Bi2MoO6) is a promising NIB anode material operating through a combined conversion/alloying mechanism. We report an operando x-ray diffraction (XRD) investigation of Bi2MoO6-based anodes over 34 (de)sodiation cycles revealing both basic operating mechanisms and potential pathways for capacity degradation. Irreversible conversion of Bi2MoO6 to Bi nanoparticles occurs through the first sodiation, allowing Bi to reversibly alloy with Na forming the cubic Na3Bi phase. Preliminary electrochemical evaluation in half-cells versus Na metal demonstrated specific capacities for Bi2MoO6 to be close to 300 mAh g−1 during the initial 10 cycles, followed by a rapid capacity decay. Operando XRD characterisation revealed that the increased irreversibility of the sodiation reactions and the formation of hexagonal Na3Bi are the main causes of the capacity loss. This is initiated by an increase in crystallite sizes of the Bi particles accompanied by structural changes in the electronically insulating Na–Mo–O matrix leading to poor conductivity in the electrode. The poor electronic conductivity of the matrix deactivates the NaxBi particles and prevents the formation of the solid electrolyte interface layer as shown by post-mortem scanning electron microscopy studies.

• Periyasamy, Manimuthu; Patra, Lokanath; Fjellvåg, Øystein; Ravindran, Ponniah; Sørby, Magnus Helgerud & Kumar, Susmit [Vis alle 8 forfattere av denne artikkelen] (2021). Effect of electron doping on the crystal structure and physical properties of an n = 3 Ruddlesden-Popper compound La4Ni3O10. ACS Applied Electronic Materials. 3(6), s. 2671–2684. doi: 10.1021/acsaelm.1c00270. Fulltekst i vitenarkiv Vis sammendrag

  The physical properties of La4Ni3O10 with a two-dimensional (2D)-like Ruddlesden–Popper-type crystal structure are extraordinarily dependent on temperature and chemical substitution. By introducing Al3+ atoms randomly at Ni sites, the average oxidation state for the two nonequivalent Ni cations is tuned and adopts values below the average of +2.67 in La4Ni3O10. La4Ni3–xAlxO10 is a solid solution for 0.00 ≤ x ≤ 1.00 and is prepared by the citric acid method, with attention paid to compositional control and homogeneity at low Al level (x) concentrations. The samples adopt a slightly distorted monoclinic structure [P21/a (Z = 4)], evidenced by peak broadening of the (117) reflection. We report a remarkable effect on the electronic properties induced by tiny amounts of homogeneously distributed Al cations, with clear correspondence between resistivity, magnetization, diffraction, and density functional theory (DFT) data. DFT shows that electronically, there is no significant difference between the nonequivalent Ni atoms and no tendency toward any Ni3+/Ni2+ charge ordering. The resistivity changes from metallic to semiconducting/insulating with increasing band gap at higher Al levels, consistent with results from DFT. The metal-to-metal (M-T-M) transition reported for La4Ni3O10, which is often interpreted as a charge density wave, is maintained until x = 0.15 Al level. However, the temperature characteristics of the resistivity change already at very low Al levels (x ≤ 0.03). A coupling of the M-T-M transition to the lattice is evidenced by an anomaly in the unit cell dimensions. Moreover, there is an excellent correlation between the resistivity and magnetization data shown by the metallic and Pauli paramagnetic regime for La4Ni3–xAlxO10 with x < 0.25. The introduction of the fixed +3 oxidation state of Al atoms randomly at the Ni sites reduces the overall oxidation state of Ni2.67+ by keeping the oxygen stoichiometry unchanged. In addition, the nonmagnetic Al3+ for Ni is likely to block Ni–O–Ni exchange pathways through −Ni–O–Al–O–Ni– fragments into the network of corner-shared octahedra with the emergence of possible short-range ferromagnetic ordering of the ferromagnetic domains/clusters that are formed due to Al substitutional disorder in a paramagnetic insulating matrix.

• Sønsteby, Henrik Hovde; Skaar, Erik; Fjellvåg, Helmer & Nilsen, Ola (2021). Effect of Subcycle Arrangement on Direct Epitaxy in ALD of LaNiO3. ACS Applied Electronic Materials. 3(1), s. 292–298. doi: 10.1021/acsaelm.0c00855. Fulltekst i vitenarkiv Vis sammendrag

  Monolithic device integration of crystalline complex oxide thin films can open for smarter and more sustainable devices in electronics and energy technology. However, the facile integration of such compounds has so far been incompatible with the present production lines for electronics. Atomic layer deposition (ALD) is already well-integrated for the production of amorphous binary oxides in electronics, but extending the technique to crystalline complex compounds has proved challenging. Herein, we show how the subcycle arrangement (i.e., the overall order of binary subcycles) plays a crucial role in the formation and quality of crystalline complex oxides by ALD, exemplified by the growth of LaNiO3. We show that an approach somewhere in between the traditional homogeneous and multilayer approaches provides the best platform for crystalline growth. Based on these results, we hypothesize that choosing multilayer unit slab thicknesses close to the interlayer distances in the target crystalline structure, while still maintaining the correct cation composition, enhances as-deposited crystallinity and in turn the functional properties. We believe this approach can be used to extend the toolbox of attainable crystalline complex oxides by ALD and establish utilization of the technique for monolithic integration of functional thin films.

• Saha, Dipankar; Kumar, Susmit; Gonano, Bruno Raymond Gino; Sjåstad, Anja Olafsen & Fjellvåg, Helmer (2021). Total Scattering Study of Local Structural Changes in MnAs1-xPx Influenced by Magnetic Interactions. Chemistry of Materials. ISSN 0897-4756. 33, s. 2576–2584. doi: 10.1021/acs.chemmater.1c00128. Fulltekst i vitenarkiv Vis sammendrag

  The interplay between fluctuations of the local structure and magnetic interactions is of great importance for phenomena like superconductivity, colossal magnetoresistance, and frustrated magnetism. Such local distortions are often subtle and difficult to probe. The metallic MnAs1–xPx (x = 0.06, 0.12, 0.18) solid solution represents a sensitive model system, currently analyzed by variable-temperature X-ray total scattering (TS). A second-order transition (Pnma to P63/mmc) occurs on heating, intimately coupled to magnetic ordering at low temperatures and a temperature-induced low- to high-spin transition for manganese. Intrinsic compositional fluctuations along with a particular magneto-volumetric coupling to low- and high-spin like states triggers formation of local high-symmetry nanosized domains in an orthorhombically deformed matrix that eventually converts globally into a high-temperature hexagonal arrangement. These features are only traced by TS analyses, whereas diffraction provides a regular second-order transition with, e.g., cationic displacements defined by Rietveld analysis acting as order parameter. The coexistence model provides good fits of the TS data and, on average, translates nicely into a continuous V(T) thermal expansion relation with maximum expansivity in the range where the TS data shows the largest difference between local (<20 Å) and intermediate-range (20–80 Å) structures. The degree of local distortions, Mn(As, P)6 octahedra inclusive, is a result of volume-dependent electronic properties as well as magnetic interactions. This study demonstrates how X-ray TS can conveniently give essential insights into the local symmetry fluctuations from the perspective of strong magnetic interactions.

• Brennhagen, Anders; Cavallo, Carmen; Wragg, David; Sottmann, Jonas; Koposov, Alexey & Fjellvåg, Helmer (2021). Understanding the (De)Sodiation Mechanisms in Na‐Based Batteries through Operando X‐Ray Methods. Batteries & Supercaps. 4(7), s. 1039–1063. doi: 10.1002/batt.202000294. Fulltekst i vitenarkiv Vis sammendrag

  Progress in the field of Na‐based batteries strongly relies on the development of new advanced materials. However, one of the main challenges of implementing new electrode materials is the understanding of their mechanisms (sodiation/desodiation) during electrochemical cycling. Operando studies provide extremely valuable insights into structural and chemical changes within different battery components during battery operation. The present review offers a critical summary of the operando X‐ray based characterization techniques used to examine the structural and chemical transformations of the active materials in Na‐ion, Na‐air and Na‐sulfur batteries during (de)sodiation. These methods provide structural and electronic information through diffraction, scattering, absorption and imaging or through a combination of these X‐ray‐based techniques. Challenges associated with cell design and data processing are also addressed herein. In addition, the present review provides a perspective on the future opportunities for these powerful techniques.

• Bernal, Fabian Leonardo Martinez; Lundvall, Fredrik; Kumar, Susmit; Hansen, Per-Anders S.; Wragg, David S. & Fjellvåg, Helmer [Vis alle 7 forfattere av denne artikkelen] (2021). Jahn-Teller active fluoroperovskites ACrF3 (A=Na+, K+): Magnetic and thermo-optical properties. PHYSICAL REVIEW MATERIALS. 5(6). doi: 10.1103/PhysRevMaterials.5.064420. Fulltekst i vitenarkiv Vis sammendrag

  Chromium (II) fluoroperovskites A CrF 3 ( A = Na + , K + ) are strongly correlated Jahn-Teller active materials at low temperatures. In this paper, we examine the role that the A -site ion plays in this family of fluoroperovskites using both experimental methods (x-ray diffraction, optical absorption spectroscopy, and magnetic fields) and density functional theory simulations. Temperature-dependent optical absorption experiments show that the spin-allowed transitions E 2 and E 3 only merge completely for A = Na at 2 K. Field-dependent optical absorption measurements at 2 K show that the oscillating strength of the spin-allowed transitions in NaCrF 3 increases with increasing applied field. Direct magneto-optical correlations suppress the spin-flip transitions for KCrF 3 below its Néel temperature. In NaCrF 3 the spin-flip transitions vanish abruptly below 9 K revealing magneto-optical correlations possibly linked to crystal structure changes. This suggests that as the long range ordering is reduced, local Jahn-Teller effects in the individual CrF 4 − 6 octahedra take control of the observed behavior. Our results show clear deviation from the pattern found for the isoelectronic A x MnF 3 + x system. The size of the A -site cation is shown to be central in dictating the physical properties and phase transitions in A CrF 3 , opening up the possibility of varying the composition to create novel states of matter with tunable properties.

• Schuler, Raphael; Bianchini, Federico; Norby, Truls Eivind & Fjellvåg, Helmer (2021). Near-Broken-Gap Alignment between FeWO4 and Fe2WO6 for Ohmic Direct p–n Junction Thermoelectrics. ACS Applied Materials & Interfaces. ISSN 1944-8244. 13(6), s. 7416–7422. doi: 10.1021/acsami.0c19341. Fulltekst i vitenarkiv
• Gonano, Bruno Raymond Gino; Fjellvåg, Øystein; Steciuk, Gwladys; Guillou, Francois; Saha, Dipankar & Pelloquin, Denis [Vis alle 7 forfattere av denne artikkelen] (2021). Tuning the Magnetically Segregated Nanolayering in Mn–Ni–As Intermetallics. Chemistry of Materials. ISSN 0897-4756. 33(8), s. 3002–3010. doi: 10.1021/acs.chemmater.1c00760. Fulltekst i vitenarkiv Vis sammendrag

  2D materials exhibiting alternating ferro- and non-ferromagnetic layers are highly sought due to their potential for applications. In this work, we demonstrate, through the use of robust methodology, the existence of single-phase, Mn1–xNixAs (0.25 ≤ x ≤ 0.50), with a unique intrinsic nanostructuring where the thickness of MnAs and NiAs nanolayers can be tuned via the composition, x. We describe a periodic variation of nanolayers of edge-sharing MnAs6 and NiAs6 octahedra layers with thicknesses ranging from 1 to 6 octahedra, 2–16 Å. These crystalline structures are modulated and fully described based on 3D electron diffraction and neutron diffraction. Moreover, by X-ray magnetic circular dichroism, an elemental and orbital-specific technique, we probe the 3d magnetic moments of Mn and Ni. We unveil that the nanosegregated MnAs and NiAs layers behave very differently, with the magnetic moment of manganese being one order of magnitude greater than that of nickel. This study opens the path to a completely new class of materials with intrinsic layers of high and low magnetization with potential applications within spintronics.

• Karthikeyan, Jayakumar; Fjellvåg, Helmer; Bundli, Silje & Sjåstad, Anja Olafsen (2021). Efficient Exfoliation of Layered Double Hydroxides; Effect of Cationic Ratio, Hydration State, Anions and Their Orientations. Materials. 14(2). doi: 10.3390/ma14020346. Fulltekst i vitenarkiv
• Pal, Santosh Kumar; Frommen, Christoph; Kumar, Susmit; Hauback, Bjørn; Fjellvåg, Helmer & Helgesen, Geir (2020). Enhancing giant magnetocaloric effect near room temperature by inducing magnetostructural coupling in Cu-doped MnCoGe. Materials & design. ISSN 0264-1275. 195, s. 1–11. doi: 10.1016/j.matdes.2020.109036. Fulltekst i vitenarkiv Vis sammendrag

  High performance magnetocaloric materials are crucial to realize the energy efficient and environment friendly magnetic cooling/refrigeration technology. We have designed Mn1-xCuxCoGe compounds possessing a giant magnetocaloric effect near room temperature. The magnetic and structural degree of freedom have been coupled by substituting Cu for Mn leading to a first-order magnetostructural phase transformation resulting in a giant magnetocaloric effect over a wide temperature window of 100 K (250–350 K). A very large entropy change value of 58 J.kg−1.K−1 corresponding to a magnetic field change of 5 T near room temperature has been obtained for Mn0.89Cu0.11CoGe exhibiting a maximum effective refrigerant capacity of 258.2 J.kg−1. The first-order magnetostructural phase transformation which is essential for the giant magnetocaloric effect has been confirmed by a combinatorial master-curve and Arrott-plot analyses. The results of giant magnetocaloric effect realized in Mn1-xCuxCoGe are comparable to or better than that of the other reported high performing materials, and this material can be of significant importance for the development of environment friendly and energy efficient cooling devices. The approach of magnetostructural coupling by tuning the structural and magnetic transitions for a giant magnetocaloric effect can also be adopted for other materials to design the best solid-state magnetic refrigerant.

• Schuler, Raphael; Norby, Truls Eivind & Fjellvåg, Helmer (2020). Defects and polaronic electron transport in Fe2WO6. Physical Chemistry, Chemical Physics - PCCP. ISSN 1463-9076. 22(27), s. 15541–15548. doi: 10.1039/d0cp01588a. Fulltekst i vitenarkiv Vis sammendrag

  We report the synthesis of phase pure Fe2WO6 and its structural characterization by high quality synchrotron X-ray powder diffraction, followed by studies of electric and thermoelectric properties as a function of temperature (200–950 °C) and pO2 (1–10−3 bar). The results are shown to be in accordance with a defect chemical model comprising formation of oxygen vacancies and charge compensating electrons at high temperatures. The standard enthalpy and entropy of formation of an oxygen vacancy and two electrons in Fe2WO6 are found to be 113(5) kJ mol−1 and 41(5) J mol−1 K−1, respectively. Electrons residing as Fe2+ in the Fe3+ host structure act as charge carriers in a small polaron conducting manner. A freezing-in of oxygen vacancies below approximately 650 °C results in a region of constant charge carrier concentration, corresponding to an iron site fraction of XFe2+ ≅ 0.03. By decoupling of mobility from conductivity, we find a polaron hopping activation energy of 0.34(1) eV and a charge mobility pre-exponential u0 = 400(50) cm2 kV−1 s−1. We report thermal conductivity for the first time for Fe2WO6. The relatively high conductivity, large negative Seebeck coefficient and low thermal conductivity make Fe2WO6 an interesting candidate as an n-type thermoelectric in air, for which we report a maximum zT of 0.027 at 900 °C.

• Sønsteby, Henrik Hovde; Skaar, Erik; Fjellvåg, Øystein; Bratvold, Jon E.; Fjellvåg, Helmer & Nilsen, Ola (2020). A foundation for complex oxide electronics -low temperature perovskite epitaxy. Nature Communications. 11(1). doi: 10.1038/s41467-020-16654-2. Fulltekst i vitenarkiv Vis sammendrag

  As traditional silicon technology is moving fast towards its fundamental limits, all-oxide electronics is emerging as a challenger offering principally different electronic behavior and switching mechanisms. This technology can be utilized to fabricate devices with enhanced and exotic functionality. One of the challenges for integration of complex oxides in electronics is the availability of appreciable low-temperature synthesis routes. Herein we provide a fundamental extension of the materials toolbox for oxide electronics by reporting a facile route for deposition of highly electrically conductive thin films of LaNiO3 by atomic layer deposition at low temperatures. The films grow epitaxial on SrTiO3 and LaAlO3 as deposited at 225 °C, with no annealing required to obtain the attractive electronic properties. The films exhibit resistivity below 100 µΩ cm with carrier densities as high as 3.6 · 1022 cm−3. This marks an important step in the realization of all-oxide electronics for emerging technological devices.

• Sønsteby, Henrik Hovde; Skaar, Erik; Bratvold, Jon Einar; Freeland, John W.; Yanguas-Gil, Angel & Elam, Jeffrey W. [Vis alle 8 forfattere av denne artikkelen] (2020). Tuning electronic properties in LaNiO3 thin films by B-site Cu-substitution. Journal of Materials Chemistry C. ISSN 2050-7526. 8(36), s. 12662–12668. doi: 10.1039/D0TC03406A. Fulltekst i vitenarkiv
• Sønsteby, Henrik Hovde; Killi, Veronica Anne-Line Kathrine; Storaas, Thomas Aarflot; Choudhuy, Devika; Elam, Jeffrey W. & Fjellvåg, Helmer [Vis alle 7 forfattere av denne artikkelen] (2020). Understanding KOtBu in atomic layer deposition–in situ mechanistic studies of the KNbO3 growth process. Dalton Transactions. ISSN 1477-9226. 49(38), s. 13233–13242. doi: 10.1039/D0DT02324H. Fulltekst i vitenarkiv
• Zacharaki, Eirini; Bremmer, Marien G.; Vajeeston, Ponniah; Kalyva, Maria Evangelou; Fjellvåg, Helmer & Kooyman, Patricia J. [Vis alle 7 forfattere av denne artikkelen] (2020). One-pot synthesis of cobalt-rhenium nanoparticles taking the unusual beta-Mn type structure. Nanoscale Advances. 2, s. 1850–1853. doi: 10.1039/d0na00097c. Fulltekst i vitenarkiv Vis sammendrag

  Using a facile one-pot colloidal method, it is now possible to obtain monodisperse Co1−xRex nanoparticles (NPs), with excellent control of Re stoichiometry for x < 0.15. Re-incorporation in terms of a solid solution stabilizes the β-Mn polymorph relative to the hcp/ccp variants of cobalt. The NPs are thermally stable up to 300 °C, which may make them attractive as model catalysts.

• Bernal, Fabian Leonardo Martinez; Sottmann, Jonas; Wragg, David; Fjellvåg, Helmer; Fjellvåg, Øystein & Drathen, Christina [Vis alle 8 forfattere av denne artikkelen] (2020). Structural and magnetic characterization of the elusive Jahn-Teller active NaCrF3. PHYSICAL REVIEW MATERIALS. 4(5). doi: 10.1103/PhysRevMaterials.4.054412. Fulltekst i vitenarkiv Vis sammendrag

  We report on the structural and magnetic properties of the elusive Jahn-Teller active compound NaCrF3, for first time synthesized in large quantities allowing detailed characterization. The crystal structure of NaCrF3 is initially described from a DFT model, which helped serve as the basis for indexing and structure determination confirmed by high-resolution synchrotron x-ray diffraction experiments. NaCrF3 adopts the triclinic space group P¯1 (isostructural with NaCuF3). Magnetometry studies at low temperature show that NaCrF3 is a weak antiferromagnet, Weiss temperature θ=−4 K. The Néel temperature is TN=21.3 K and the paramagnetic moment μ=4.47μB is in accordance with the theoretical S=2. Field-dependent measurements between 2 and 12 K unveil the onset of metamagnetic behavior. Our experiments revealed a weakly canted A-type magnetic structure observed by neutron powder diffraction, with a magnetic propagation vector (1/2,1/2,0) and a magnetic moment of 3.51μB at 1.5 K. Our results shed further light on the Jahn-Teller effects and strong correlations as a function of A-ion size in the family ACrF3.

• Arstad, Bjørnar; Blom, Richard; Håkonsen, Silje Fosse; Pierchala, Joanna; Cobden, Paul & Lundvall, Fredrik [Vis alle 10 forfattere av denne artikkelen] (2020). Synthesis and Evaluation of K-Promoted Co3-xMgxAl-Oxides as Solid CO2 Sorbents in the Sorption-Enhanced Water−Gas Shift (SEWGS) Reaction. Industrial & Engineering Chemistry Research. ISSN 0888-5885. 59(40), s. 17837–17844. doi: 10.1021/acs.iecr.0c02322. Fulltekst i vitenarkiv Vis sammendrag

  Hydrogen is essential in a variety of large-scale chemical processes. As a carbon-free energy carrier, hydrogen has a potential for wide use within power production and transportation. However, most of the recent production methods involve the release of CO2 as a by-product. Hence, decarbonization of hydrogen production is one step to reduce CO2 emission into the Earth’s atmosphere. Several process schemes have been suggested for low-carbon emission production of hydrogen. In this work, we show how to improve solid sorbents for the sorption-enhanced water–gas shift (SEWGS) process, which is a process that exploits a solid sorbent in the water–gas shift reactor to capture CO2 in situ and drive the process toward an improved hydrogen yield. We report herein a series of CoxMg3-xAl materials based on hydrotalcites, promoted with various loadings of K. The materials have been characterized by BET, XRD, and NMR and tested for their CO2 adsorption performance in three adsorption–desorption cycles in a lab-scale fixed-bed reactor (20–22 bar, CO2 + steam as reactant gas, and isothermal conditions at 375 and 400 °C). The most promising material was subjected to a long-term test (120 adsorption–desorption cycles at similar conditions). This test indicates that a K-promoted Co1.5Mg1.5Al (22 wt % of added K2CO2 to the oxide) material has a higher cyclic capacity for CO2 than standard reference cases. We have estimated that the volumetric capacity (in mol/L unit) of this sorbent will be 23–26% higher than a standard reference material at 400 °C and 30–39% higher at 375 °C. This would, in fixed-bed columns, lead to significant reduction in the needed column volumes in the final process and reduce costs.

• El-Kharbachi, Abdelouahab; Wind, Julia; Ruud, Amund; Høgset, Astrid Bygdås; Nygård, Magnus Moe & Zhang, Junxian [Vis alle 14 forfattere av denne artikkelen] (2020). Pseudo-ternary LiBH4-LiCl-P2S5 system as structurally disordered bulk electrolyte for all-solid-state lithium batteries. Physical Chemistry, Chemical Physics - PCCP. ISSN 1463-9076. 22(25), s. 13872–13879. doi: 10.1039/d0cp01334j. Fulltekst i vitenarkiv Vis sammendrag

  The properties of the mixed system LiBH4–LiCl–P2S5 are studied with respect to all-solid-state batteries. The studied material undergoes an amorphization upon heating above 60 °C, accompanied with increased Li+ conductivity beneficial for battery electrolyte applications. The measured ionic conductivity is ∼10−3 S cm−1 at room temperature with an activation energy of 0.40(2) eV after amorphization. Structural analysis and characterization of the material suggest that BH4 groups and PS4 may belong to the same molecular structure, where Cl ions interplay to accommodate the structural unit. Thanks to its conductivity, ductility and electrochemical stability (up to 5 V, Au vs. Li+/Li), this new electrolyte is successfully tested in battery cells operated with a cathode material (layered TiS2, theo. capacity 239 mA h g−1) and Li anode resulting in 93% capacity retention (10 cycles) and notable cycling stability under the current density ∼12 mA g−1 (0.05C-rate) at 50 °C. Further advanced characterisation by means of operando synchrotron X-ray diffraction in transmission mode contributes explicitly to a better understanding of the (de)lithiation processes of solid-state battery electrodes operated at moderate temperatures.

• Gonano, Bruno Raymond Gino; Fjellvåg, Øystein; Steciuk, Gwladys; Saha, Dipankar; Pelloquin, Denis & Fjellvåg, Helmer (2020). Exotic Compositional Ordering in Manganese–Nickel–Arsenic (Mn-Ni-As) Intermetallics. Angewandte Chemie International Edition. ISSN 1433-7851. 132(50), s. 22568–22573. doi: 10.1002/anie.202006135. Fulltekst i vitenarkiv Vis sammendrag

  In this work we benefited from recent advances in tools for crystal‐structure analysis that enabled us to describe an exotic nanoscale phenomenon in structural chemistry. The Mn0.60Ni0.40As sample of the Mn1−xNixAs solid solution, exhibits an incommensurate compositional modulation intimately coupled with positional modulations. The average structure is of the simple NiAs type, but in contrast to a normal solid solution, we observe that manganese and nickel segregate periodically at the nano‐level into ordered MnAs and NiAs layers with thickness of 2–4 face‐shared octahedra. The detailed description was obtained by combination of 3D electron diffraction, scanning transmission electron microscopy, and neutron diffraction. The distribution of the manganese and nickel layers is perfectly described by a modulation vector q=0.360(3) c*. Displacive modulations are observed for all elements as a consequence of the occupational modulation, and as a means to achieve acceptable Ni–As and Mn–As distances. This modulated evolution of magnetic MnAs and non‐magnetic NiAs‐layers with periodicity at approximately 10 Å level, may provide an avenue for spintronics.

• Duparc, Marion Jeanne Louise; Sønsteby, Henrik Hovde; Nilsen, Ola; Sjåstad, Anja Olafsen & Fjellvåg, Helmer (2020). Atomic Layer Deposition of GdCoO3 and Gd0.9Ca0.1CoO3. Materials. 13(1), s. 1–10. doi: 10.3390/ma13010024. Fulltekst i vitenarkiv Vis sammendrag

  Thin films of the catalytically interesting ternary and quaternary perovskites GdCoO3 and Gd0.9Ca0.1CoO3 are fabricated by atomic layer deposition using metal β-diketonates and ozone as precursors. The resulting thin films are amorphous as deposited and become single-oriented crystalline on LaAlO3(100) and YAlO3(100/010) after post-annealing at 650 °C in air. The crystal orientations of the films are tunable by choice and the orientation of the substrate, mitigated through the interface via solid face epitaxy upon annealing. The films exhibit no sign of Co2+. Additionally, high-aspect-ratio Si(100) substrates were used to document the suitability of the developed process for the preparation of coatings on more complex, high-surface-area structures. We believe that coatings of GdCoO3 and Gd1−xCaxCoO3 may find applications within oxidation catalysis.

• Sønsteby, Henrik Hovde; Bratvold, Jon Einar; Killi, Veronica Anne-Line Kathrine; Choudhury, Devika; Elam, Jeffrey W. & Fjellvåg, Helmer [Vis alle 7 forfattere av denne artikkelen] (2020). tert-butoxides as precursors for atomic layer deposition of alkali metal containing thin films. Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. ISSN 0734-2101. 38, s. 1–15. doi: 10.1116/6.0000589. Fulltekst i vitenarkiv Vis sammendrag

  Alkali metal containing materials have become increasingly attractive in a world hunting for sustainable energy materials and green functional devices. Lithium- and sodium battery technology, lead-free piezo- and ferroelectric devices, and record-breaking alkali doped tandem perovskite solar cells are among the applications where alkali metal-containing thin films get increasing attention. Atomic layer deposition (ALD) is one of the enabling thin film deposition techniques that offer chemical and geometrical versatility to realize the implementation of such thin films on an applicable scale. The drawback has until recently been a lack of available precursor chemistry that offers self-limiting growth that is fundamental to ALD. The alkali metal tert-butoxides have been shown to exhibit the necessary properties to facilitate saturating growth for Li-, Na-, K-, and Rb-containing compounds. However, the behavior of the tert-butoxides in ALD-growth has been considered difficult to unravel, with processes exhibiting limited control and low reproducibility. Very little has been reported on trends in reaction mechanisms as the mass of the alkali metal increases. Herein, we summarize the existing literature on the use of alkali metal tert-butoxides as precursors in ALD. We consider differences in the structure and behavior of the tert-butoxides as the alkali metal cation becomes heavier. In addition, we present precursor synthesis routes and key information on precursor structure, stability, and mechanistic behavior. Finally, we provide the first ever report of Cs-containing films by ALD to complement previous work on its lighter counterparts.

• Dietzel, Pascal D.C.; Georgiev, Peter A.; Frøseth, Morten; Johnsen, Rune E.; Fjellvåg, Helmer & Blom, Richard (2020). Effect of larger pore size on the sorption properties of isoreticular metal–organic frameworks with high number of open metal sites. Chemistry - A European Journal. ISSN 0947-6539. 26(59), s. 13523–13531. doi: 10.1002/chem.202001825. Fulltekst i vitenarkiv Vis sammendrag

  Four isostructural CPO‐54‐M metal‐organic frameworks based on the larger organic linker 1,5‐dihydroxynaphthalene‐2,6‐dicarboxylic acid and divalent cations (M=Mn, Mg, Ni, Co) are shown to be isoreticular to the CPO‐27 (MOF‐74) materials. Desolvated CPO‐54‐Mn contains a very high concentration of open metal sites, which has a pronounced effect on the gas adsorption of N2, H2, CO2 and CO. Initial isosteric heats of adsorption are significantly higher than for MOFs without open metal sites and are slightly higher than for CPO‐27. The plateau of high heat of adsorption decreases earlier in CPO‐54‐Mn as a function of loading per mole than in CPO‐27‐Mn. Cluster and periodic density functional theory based calculations of the adsorbate structures and energetics show that the larger adsorption energy at low loadings, when only open metal sites are occupied, is mainly due to larger contribution of dispersive interactions for the materials with the larger, more electron rich bridging ligand.

• Bernal, Fabian Leonardo Martinez; Gonano, Bruno Raymond Gino; Lundvall, Fredrik; Wragg, David; Fjellvåg, Helmer & Veillon, Fabien [Vis alle 8 forfattere av denne artikkelen] (2020). Canted antiferromagnetism in high-purity NaFeF3 prepared by a novel wet-chemical synthesis method. PHYSICAL REVIEW MATERIALS. 4(11). doi: 10.1103/PhysRevMaterials.4.114412. Fulltekst i vitenarkiv Vis sammendrag

  We report a synthesis method, and structural and magnetic characterization, for the fluoroperovskite NaFeF3. We have developed a wet-chemical method that allows preparation of large volumes of air-sensitive fluoroperovskites with high purity. NaFeF3 has a Néel temperature (TN) of 90 K and a Weiss constant (θ) of −124 K, corresponding to dominant antiferromagnetic interactions. Below TN, a slight difference is observed between zero-field and field-cooled samples, indicating spin canting and weak ferromagnetism. AC magnetometry supports that weak ferromagnetism is inherent to NaFeF3 and not due to an impurity. From powder neutron diffraction data, we describe the magnetic structure precisely as a weakly canted G-type (magnetic space group Pn′ma′). A ferromagnetic component is allowed in Pn′ma′, however, this component is too small to be confirmed on the basis of powder neutron diffraction data and may be absent in zero magnetic fields.

• Periyasamy, Manimuthu; Fjellvåg, Øystein; Fjellvåg, Helmer & Sjåstad, Anja Olafsen (2020). Coupling of magnetoresistance switching and glassy magnetic state at the metal–insulator transition in Ruddlesden-Popper manganite Ca4Mn3O10. Journal of Magnetism and Magnetic Materials. ISSN 0304-8853. 511, s. 1–8. doi: 10.1016/j.jmmm.2020.166949. Fulltekst i vitenarkiv Vis sammendrag

  Electronic phase separation is a frequent ingredient of the physics of manganites for which the colossal magnetoresistance (CMR) phenomenon is at debate. We investigate the switchable magnetoresistance (MR) effect of the Ruddelsden-Popper (RP) oxide Ca4Mn3O10 (CMO), which shows a metal-to-insulator transition (MIT) at 70 K (TMIT). The static (DC) and dynamic (AC) magnetic susceptibilities of CMO indicate an unusual magnetic phase coexistence below the Néel temperature (TN) i.e., at TMIT, which we describe as a glassy magnetic behavior. Furthermore, by means of synchrotron X-ray diffraction, we observed a structural anomaly directly associated with the MIT. Hence, the onset of glassiness correlates with a slight structural distortion. Interestingly, CMO displays a large positive magneto-resistance (PMR) effect (up to 550% at 4 K) in the metallic state below TMIT, while a small negative magneto-resistance (NMR) effect (-3.5% at 112 K) in the insulating region above TMIT. At TMIT, the PMR switches to NMR behavior. PMR is probably induced via enhancing the exchange interaction by localization of itinerant moment resulting in formation of magnetic polarons, whereas NMR is possibly due to reduced electron-spin scattering. This appears to be a special type of magnetoresistance, different from regular observations, and may open a new avenue in searching for exotic phenomena in manganese-based CMR materials.

• Dietzel, Pascal D.C.; Blom, Richard & Fjellvåg, Helmer (2020). Variability in the formation and framework polymorphism of metal-organic frameworks based on yttrium(III) and the bifunctional organic linker 2,5-dihydroxyterephthalic acid. Zeitschrift für Anorganische und Allgemeines Chemie. ISSN 0044-2313. 647(1), s. 15–25. doi: 10.1002/zaac.202000276. Fulltekst i vitenarkiv Vis sammendrag

  Depending on the solvothermal reaction conditions, we obtained three different metal‐organic frameworks with yttrium(III) as metal component and 2,5‐dihdyroxyterepthalic acid (H4dhtp) as bifunctional organic linker: Y2(H2dhtp)3(dmf)4·(dmf)2 (CPO‐29) contains dinuclear, paddle‐wheel like inorganic secondary building units (SBUs) connected by the organic linker to a network with α‐Po topology, while Y2(H2dhtp)(dhtp)(dmf)2 (CPO‐30) and Y2(H2dhtp)(dhtp)(dmf)2(H2O)2·(H2O)4 (CPO‐31) contain one‐dimensional inorganic SBUs that differ in how the half‐ and fully deprotonated ligands are connected to and arranged around them. Only the carboxylic acid groups of the organic linker are deprotonated in CPO‐29, while CPO‐30 and CPO‐31 contain both 2,5‐dihydroxyterephthalate (H2dhtp2–) linkers and fully deprotonated 2,5‐dioxidoterephthalate (dhtp4–) linkers. All three compounds contain large volumes filled with solvent, but we were able to demonstrate permanence of porosity only for CPO‐30. Variable temperature powder X‐ray diffraction reveals that CPO‐29 and CPO‐31 undergo discontinuous phase transitions upon heating, and the flexibility of the framework structure indicated by these might be the reason for the inability to access the pore volume. Desolvated CPO‐30 and CPO‐31 are polymorphs, whose network structures differ in whether the H2dhtp2– and dhtp4– linkers are located in cis or trans arrangement around the inorganic SBU.

• Hu, Yang; Miikkulainen, Ville; Mizohata, Kenichiro; Norby, Truls Eivind; Nilsen, Ola & Fjellvåg, Helmer (2020). Ionic conductivity in LixTaOy thin films grown by Atomic Layer Deposition (ALD). Electrochimica Acta. ISSN 0013-4686. 361. doi: 10.1016/j.electacta.2020.137019. Fulltekst i vitenarkiv Vis sammendrag

  The material system Li-Ta-O is a promising candidate for thin-film solid-state electrolytes in Li-ion batteries. In the present study, we have varied the Li content x in LixTaOy thin films grown by atomic layer deposition (ALD) with the aim of improving the Li-ion conductivity. The amorphous films were grown at 225 °C on insulating sapphire and on conductive Ti substrates using tantalum ethoxide (Ta(OEt)5), lithium tert-butoxide (LiOtBu) and water as reactants. The film composition was determined by time-of-flight elastic recoil detection analysis (TOF-ERDA), displaying an almost linear relationship between the pulsed and deposited Li content. The ionic conductivities were determined by in-plane and cross-plane AC measurements, exhibiting an Arrhenius-type behaviour and comparatively weak thickness-dependence. Increasing Li content x from 0.32 to 0.98 increases the film conductivity by two orders of magnitude while higher Li content x = 1.73 results in decreased conductivity. A room-temperature conductivity σRT of ~10−8 S cm−1 is obtained for a 169 nm thick Li0.98TaOy film. The evolution of conductivity and activation energy suggests a competing effect between the concentration and the mobility of mobile Li ions when more Li are incorporated. The compositional dependence of Li transport mechanism is discussed.

• Kalantzopoulos, Georgios N.; Lundvall, Fredrik; Thorshaug, Knut; Lind, Anna Maria; Vajeeston, Ponniah & Dovgaliuk, Iurii [Vis alle 9 forfattere av denne artikkelen] (2020). Factors Determining Microporous Material Stability in Water: The Curious Case of SAPO-37. Chemistry of Materials. ISSN 0897-4756. 32(4), s. 1495–1505. doi: 10.1021/acs.chemmater.9b04510. Fulltekst i vitenarkiv Vis sammendrag

  Silicoaluminophosphates (SAPOs) are a special class of zeolites that, due to their acidic and shape-selective properties, play a major role in ion exchange and separation processes and in crude oil cracking. SAPO-37 has the faujasite (FAU) topology same as zeolites X and Y, which are involved in more than 40% of the total crude oil conversion worldwide. A critical parameter that promotes detrimental structural transformations in SAPOs during real-life applications is the presence of humidity. In this study, we employ a multidisciplinary approach combining in situ synchrotron radiation powder X-ray diffraction (SR-PXRD), water adsorption, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and density functional theory (DFT) calculations to describe the mechanism and reveal the reasons why SAPO-37 collapses upon contact with humidity below 345 K. SR-PXRD revealed that the sodalite (SOD) cages (subunits of the FAU structure) have the strongest affinity to water during hydration below 345 K. Furthermore, below 345 K, the faujasite framework takes up an order of magnitude more water molecules than at temperatures above 345 K. DRIFTS confirmed the presence of Si−OH and P−OH surface structural defects that act as hydration centers, accelerating the loss of a long-range order. Finally, DFT calculations showed that the enthalpy of water adsorption in the sodalite cage and the faujasite supercage is −212 and −13 kJ/mol, respectively. The results presented in this work are highly topical for understanding the effect of water on the frameworks of the SAPO microporous catalysts family. The notorious instability of SAPO-37 is the result of the accumulative contribution of topological, physical, and chemical effects, leading to an array of rapidly evolving cascading effects. Our work shows how advancements in SR-PXRD methodology and hardware give new insight into highly dynamic features previously difficult to observe. In addition, this work introduces the conceptual insight that nonhomogeneous sorption of molecular species will induce dynamic features with dramatic consequences at both molecular and atomic levels. This is a highly impactful factor opening research paths for further work within catalysis, porous material design and chemistry, and sorption reactions and processes.

• Fjellvåg, Øystein; Ruud, Amund; Sønsteby, Henrik Hovde; Nilsen, Ola & Fjellvåg, Helmer (2020). Crystallization, Phase Stability, and Electrochemical Performance of β-MoO3 Thin Films. Crystal Growth & Design. ISSN 1528-7483. 20(6), s. 3861–3866. doi: 10.1021/acs.cgd.0c00156. Fulltekst i vitenarkiv Vis sammendrag

  Insight into the crystallization process of functional materials deposited as thin films in an amorphous state is essential when phase pure samples of specific polymorphs are sought for utilization of performance aspects or fundamental studies. In this work, we report on controlled crystallization of α- and β-MoO3 polymorphs from amorphous MoO3 thin films produced by atomic layer deposition (ALD). The MoO3 films are amorphous as deposited. Our experiments show that β-MoO3 starts to crystallize from amorphous MoO3 at a surprisingly low temperature of 185 °C, only 20 °C above the adopted ALD deposition temperature. Thin films of the β-MoO3 polymorph start to slowly convert into α-MoO3 at temperatures above 300 °C; however, long holding times are required to obtain phase pure products. High-quality thin film samples (electrodes) were characterized by electrochemical cycling toward lithium, demonstrating that α- and β-MoO3 transfers into the same chemical state during cycling and that amorphous MoO3 undergoes less change upon cycling.

• Bratvold, Jon E.; Fjellvåg, Helmer & Nilsen, Ola (2020). Phase and Orientation Control of NiTiO3 Thin Films. Materials. 13(1), s. 1–19. doi: 10.3390/ma13010112. Fulltekst i vitenarkiv Vis sammendrag

  Subtle changes in the atomic arrangement of NiTiO3 in the ilmenite structure affects its symmetry and properties. At high temperatures, the cations are randomly distributed throughout the structure, resulting in the corundum structure with R−3c symmetry. Upon cooling, the cations order in alternating layers along the crystallographic c axis, resulting in the ilmenite structure with R−3 symmetry. Related to this is the R3c symmetry, where the cations alternate both perpendicularly and along the c axis. NiTiO3 with the latter structure is highly interesting as it exhibits ferroelectric properties. The close relationship between structure and properties for ilmenite-related structures emphasizes the importance of being able to control the symmetry during synthesis. We show that the orientation and symmetry of thin films of NiTiO3 formed by atomic layer deposition (ALD) can be controlled by choice of substrate. The disordered phase (R−3c), previously only observed at elevated temperatures, have been deposited at 250 °C on α-Al2O3 substrates, while post-deposition annealing at moderate temperatures (650 °C) induces ordering (R−3). We have in addition explored the symmetry and epitaxial orientation obtained when deposited on substrates of LaAlO3(100), SrTiO3(100) and MgO(100). The presented work demonstrates the possibilities of ALD to form metastable phases through choice of substrates.

• Li, Xinyu; Bianchini, Federico; Wind, Julia; Pettersen, Christine; Vajeeston, Ponniah & Wragg, David [Vis alle 7 forfattere av denne artikkelen] (2020). Insights into Crystal Structure and Diffusion of Biphasic Na2Zn2TeO6. ACS Applied Materials & Interfaces. ISSN 1944-8244. 12(25), s. 28188–28198. doi: 10.1021/acsami.0c05863. Fulltekst i vitenarkiv Vis sammendrag

  The layered oxide Na2Zn2TeO6 is a fast Na+ ion conductor and a suitable candidate for application as a solid-state electrolyte. We present a detailed study on how synthesis temperature and Na-content affect the crystal structure and thus the Na+ ion conductivity of Na2Zn2TeO6. Furthermore, we report for the first time an O′3-type phase for Na2Zn2TeO6. At a synthesis temperature of 900 °C, we obtain a pure P2-type phase, providing peak performance in Na+ ion conductivity. Synthesis temperatures lower than 900 °C produce a series of mixed P2 and O′3-type phases. The O′3 structure can only be obtained as a pure phase by substituting Li on the Zn-sites to increase the Na-content. Thorough analysis of synchrotron data combined with computational modeling indicates that Li enters the Zn sites and, consequently, the amount of Na in the structure increases to balance the charge according to the formula Na2+xZn2–xLixTeO6 (x = 0.2–0.5). Impedance spectroscopy and computational modeling confirm that reducing the amount of the O′3-type phase enhances the Na+ ion mobility.

• Varunaa, R; Fjellvåg, Helmer & Ponniah, Ravindran (2019). Zinc substituted MgH2 - a potential material for hydrogen storage applications. International Journal of Hydrogen Energy. ISSN 0360-3199. 44(26), s. 13632–13646. doi: 10.1016/j.ijhydene.2019.04.016. Fulltekst i vitenarkiv
• Kiruthika, S.; Fjellvåg, Helmer & Ravindran, P. (2019). Amphoteric behavior of hydrogen (H+1 and H-1) in complex hydrides from van der Waals interaction-including ab initio calculations. Journal of Materials Chemistry A. ISSN 2050-7488. 7(11), s. 6228–6240. doi: 10.1039/c8ta11685g. Fulltekst i vitenarkiv
• Bratvold, Jon E.; Sønsteby, Henrik Hovde; Nilsen, Ola & Fjellvåg, Helmer (2019). Control of growth orientation in as-deposited epitaxial iron-rich nickel ferrite spinel. Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. ISSN 0734-2101. 37(2), s. 1–7. doi: 10.1116/1.5082012. Fulltekst i vitenarkiv Vis sammendrag

  The authors here report epitaxial growth of the ferrimagnet NixFe3–xO4 (NFO) by atomic layer deposition at low temperatures. Films grow epitaxially at a reactor temperature of 250 °C and require no further postannealing treatment. (100)-, (110)-, and (111)-oriented thin films of NFO are attainable by using MgO (100)/LaAlO3 (100), LaAlO3 (110), and Al2O3 (001) substrates, respectively. This means that the direction of the easy axis of magnetization relative to the film surface can be selected by the choice of substrate. The authors carefully study nucleation onset and growth dynamics by varying the number of cycles used to deposit the films. This shows that the initial growth is mostly inhibited by the Fe-species, creating a small composition gradient for the first few nanometers of the film. The growth rate varies as a result of the choice of substrate, explained by the direction of the close packed layers in the film. The authors believe that the ability to deposit epitaxial ferrimagnetic films with angstrom thickness control at low temperatures will make a major impact on the possibility to grow multiferroic stacks and on low temperature monolithic device integration where a spontaneously magnetized sample is required.

Se alle arbeider i NVA

• Kumar, Abhishek; Fjellvåg, Helmer; Deledda, Stefano; Hauback, Bjørn Christian & Sjåstad, Anja Olafsen (2026). Composition-Tuned Phase Evolution in BCC (TiVZrNb)1-xMx HEAs (M = Ni, Fe, Mn, Co, Cr). Fulltekst i vitenarkiv
• Skurtveit, Amalie; Jr, Andrew Pastusic; Brennhagen, Anders; Maddar, Faduma M.; Mohn, Chris Erik & Karmakar, Abhoy [Vis alle 13 forfattere av denne artikkelen] (2026). Hidden Na5SbSe Unlocks Sb2Se3 Anodes for Na-Ion Batteries. Fulltekst i vitenarkiv Vis sammendrag

  Antimony selenide (Sb2Se3) is a promising alternative to commonly used NIB anodes, storing 12 Na+ per formula unit, to achieve a capacity of 670 mAh g-1. The (de)sodiation mechanism is very complex: the material progresses through multiple phases when interacting with Na+, including intercalation, con-version, and alloying reactions. These multiple chemical transformations may kinetically limit the material at high rates, however, Sb2Se3 can deliver almost 150 mAh g-1 at 5000 mA g-1. Understanding this unusual behavior is important not only to rationalize the electrochemical performance, but also for the design of other materials capable of working at high (dis)charge rates. Meanwhile, subtle phase transitions or transformations of active battery materials can be traced in operando X-ray diffraction (XRD), which enables the extraction of real-time information about potentially hidden mechanistic pathways, including the discovery of new materials. In this study, Na5SbSe was discovered as the main sodiated product of Sb2Se3 anodes. Na5SbSe is predominantly composed of Na-ions and has a highly flexible composition (breathing effect) that unlocks the exceptional high rate performance of Sb2Se3 anodes

• Skurtveit, Amalie; Jr, Andrew Pastusic; Brennhagen, Anders; Maddar, Faduma M.; Mohn, Chris Erik & Karmakar, Abhoy [Vis alle 13 forfattere av denne artikkelen] (2026). Material's Discovery via Synchrotron Operando XRD and XAS: the Case of Sb2Se3 Anodes for Na-Ion Batteries. Fulltekst i vitenarkiv Vis sammendrag

  Antimony selenide (Sb2Se3) is a promising alternative to commonly used NIB anodes, storing 12 Na+ per formula unit, to achieve a capacity of 670 mAh g-1. The (de)sodiation mechanism is very complex: the material progresses through multiple phases when interacting with Na+, including intercalation, con-version, and alloying reactions. These multiple chemical transformations may kinetically limit the material at high rates, however, Sb2Se3 can deliver almost 150 mAh g-1 at 5000 mA g-1. Understanding this unusual behavior is important not only to rationalize the electrochemical performance, but also for the design of other materials capable of working at high (dis)charge rates. Meanwhile, subtle phase transitions or transformations of active battery materials can be traced in operando X-ray diffraction (XRD), which enables the extraction of real-time information about potentially hidden mechanistic pathways, including the discovery of new materials. In this study, operando XRD and X-ray absorption spectroscopy (XAS) were employed to unravel the mysteries of the (de)sodiation mechanism of Sb2Se3 anodes. Complete elucidation of the reaction pathway was further enabled by solid-state NMR, ex situ work, and ab initio simulations.

• Kjeldby, Snorre Braathen; Olsen, Vegard Skiftestad; Frodason, Ymir Kalmann; Zacharaki, Eirini; Cooil, Simon Phillip & Vines, Lasse [Vis alle 12 forfattere av denne artikkelen] (2025). Thin film top cell absorbers for multijunction solar cells. Fulltekst i vitenarkiv
• Brennhagen, Anders; Skurtveit, Amalie; Skautvedt, Casper; Cavallo, Carmen; Wragg, David Stephen & Vajeeston, Ponniah [Vis alle 9 forfattere av denne artikkelen] (2024). Revealing the (de)sodiation mechanisms of Bi-metallates through operando X-ray characterisation. Fulltekst i vitenarkiv Vis sammendrag

  Na-ion batteries is entering the battery market as an alternative to Li-ion batteries. To improve their overall performance it is crucial to develop new types of anode materials with high capacity and long cycle life. Materials combing conversion and alloying mechanisms (CAMs) are promising anodes with their high capacity, but obtaining good cycling stability is still challenging. A comprehensive understanding of the cycling and degradation mechanism of these materials is crucial to improve their performance. Bi-metallates, with a general formula of Bi−TM−O (TM = transition metal) is a group of ternary CAMs. Their general cycling mechanism consists of an irreversible conversion reaction forming nanoparticles of Bi-metal embedded in a Na-TM-O matrix during the first sodiation, followed by a reversible two-step alloying reaction forming Na3Bi with NaBi as an intermediate phase. In this work, we have used operando X-ray diffraction (XRD), pair distribution function (PDF) analysis and X-ray absorption spectroscopy (XAS) to investigate the desodiation mechanisms of Bi2MoO6 and BiFeO3. Through this work, we discovered that Bi2MoO6 forms the cubic version of Na3Bi (c-Na3Bi) while BiFeO3 forms hexagonal Na3Bi (h-Na3Bi) in addition to c-Na3Bi during the first sodiation. In the desodiated state, the Bi-particles are partially oxidised, while still maintaining the Bi-metal structure, indicating that it is only the Bi atoms at the interface between the Bi nanoparticles and the Na−TM−O matrix that is oxidised. During cycling the NaxBi particles grow larger thus increasing the distance between them and increasing the impedance in the material. This is considered to be the main driver for the capacity degradation that was observed during the first 20 cycles. The operando XAS data also revealed that Mo6+ in Bi2MoO6 does not change oxidation state during cycling, but changes coordination between tetrahedral and distorted octahedral coordination during cycling. The cycling and degradation mechanisms of Bi2MoO6 is summarised in Figure 1.

• Brennhagen, Anders; Skautvedt, Casper; Cavallo, Carmen; Skurtveit, Amalie; Wragg, David Stephen & Koposov, Alexey [Vis alle 9 forfattere av denne artikkelen] (2023). Revealing the Cycling and Degradation Mechanism of Bi-Metallates as Anode Materials fro Na-ion Batteries. Fulltekst i vitenarkiv Vis sammendrag

  Na-ion batteries (NIBs) represent an emerging alternative to modern Li-ion batteries. To improve the overall performance of NIBs it is crucial to develop new types of anode materials with high capacity and long cycle life. Among the anode candidates, materials operating under combined conversion and alloying mechanisms (CAMs) are considered as promising. They are capable of delivering high capacity, however, despite theoretical predictions the cycling stability is still below expectations. Therefore, a comprehensive understanding of the cycling and degradation mechanisms of CAMs is crucial to improve their performance. However, these materials undergo multiple chemical transformations, which often involves formation of amorphous and nanocrystalline phases during cycling. Such chemical complexity substantially impedes their full characterization and understanding. Bismuth metallates, with a general formula of Bi-TM-O (TM = transition metal), is a group of ternary CAMs. Their general cycling mechanism consists of an irreversible conversion reaction, which leads to formation of Bi nanoparticles embedded in a Na-TM-O matrix during the first sodiation. This is followed by a reversible two-step alloying reaction leading to formation of Na3Bi with NaBi as an intermediate phase. In this work, we have used operando X-ray diffraction over the course of ∼30 cycles to study the cycling and degradation mechanisms of two CAMs - Bi2MoO6 and BiFeO3. With the help of ex situ X-ray absorption spectroscopy and pair distribution function (PDF) analysis, we have revealed new insights into these materials in addition to confirming the general cycling mechanism. Both materials form the cubic phase of Na3Bi (c-Na3Bi) in the sodiated state with some noticeable differences. While, the hexagonal Na3Bi (h-Na3Bi) phase starts to form after 10 cycles in Bi2MoO6, a combination of c-Na3Bi and h-Na3Bi can be observed after the first cycle in BiFeO3. The formation of c-Na3Bi is kinetically favorable in these systems, while h-Na3Bi is the more thermodynamically stable and, therefore, forms later in the cycling process. Both materials shows three distinct regions of capacity degradation, where the first is initiated by a growth in the crystallite sizes of Bi-particles. This is followed by a disappearance of the Bi-phase as the second step. The last degradation phase is related to the disappearance of the NaBi-phase and locking the system in the sodiated state. The present talk will discuss the mechanisms of these degradation steps in view of the future development of CAMs in greater details.

• Sottmann, Jonas; Ruud, Amund; Fjellvåg, Øystein Slagtern; Vaughan, Gavin B. M.; Michel, Marco Di & Fjellvåg, Helmer [Vis alle 9 forfattere av denne artikkelen] (2023). Erratum: 5D total scattering computed tomography reveals the full reaction mechanism of a bismuth vanadate lithium ion battery anode (Phys. Chem. Chem. Phys. (2022) 24 (27075-27085) DOI: 10.1039/D2CP03892G). Physical Chemistry, Chemical Physics - PCCP. ISSN 1463-9076. doi: 10.1039/d3cp90076b. Fulltekst i vitenarkiv
• Brennhagen, Anders; Cavallo, Carmen; Wragg, David Stephen; Koposov, Alexey; Ponniah, Vajeeston & Sjåstad, Anja Olafsen [Vis alle 7 forfattere av denne artikkelen] (2023). Tracing the (De)sodiation of Bi2MoO6 Through Good and Bad Times With Operando XRD. Fulltekst i vitenarkiv Vis sammendrag

  Na-ion batteries (NIBs) generally have slightly lower energy densities than Li-ion batteries (LIBs), but the abundancy of Na could make NIBs cheaper and more sustainable. The search for new anode materials is essential for further development of NIBs. Anode materials combining conversion and alloying mechanisms (CAMs) are promising because of their high capacity and high rate capabilities, but their complex cycling mechanisms involving amorphous phases are challenging to study [1]. Bi2MoO6 is an example of a CAM that has high initial capacity, but struggles with cycling stability and undergoes major changes during cycling. Detailed understanding of its cycling and degradation mechanism will hopefully enable us to improve the cycling stability of Bi2MoO6 and CAMs in general. Therefore, we have studied Bi2MoO6 using a combination of operando X-ray diffraction (Fig. 1), pair distribution function, X-ray absorption spectroscopy and transmission electron microscopy [3]. This revealed that during the first sodiation, an irreversible conversion of Bi2MoO6 occurs, creating Bi nanoparticles embedded in an amorphous Na-Mo-O matrix. The Bi particles then reversibly alloy with Na, forming cubic Na3Bi leading to a specific capacity close to 300 mAh g−1 for the 10 first cycles. During these cycles, we observe a growth in the crystallite size of the alloying particles, which is probably the leading cause of the rapid capacity decay after cycle 10 where the sodiation of Bi becomes irreversible leaving several inactive Na3Bi particles.

• Brennhagen, Anders; Cavallo, Carmen; Wragg, David Stephen; Koposov, Alexey; Ponniah, Vajeeston & Sjåstad, Anja Olafsen [Vis alle 7 forfattere av denne artikkelen] (2022). Tracing the (De)sodiation of Bi2MoO6 Through Good and Bad Times With Operando XRD. Fulltekst i vitenarkiv Vis sammendrag

  Anode materials combining conversion and alloying mechanisms (CAMs) are promising for Na-ion batteries, but their complex cycling mechanisms are challenging to study. Understanding the (de)sodiation mechanism is crucial and, in several cases, requires advanced synchrotron characterization. Operando synchrotron studies are generally limited to one or two sodiation cycles, which are not fully comprehensive for CAMs. Therefore, herein, we studied the sodiation and desodiation of Bi2MoO6-based anodes with laboratory-based operando X-ray diffraction exceeding more than 30 cycles to have a complete overview of our CAM’s mechanism. This revealed important aspects of the cycling and degradation mechanisms in the material. During the first sodiation, an irreversible conversion of Bi2MoO6 occurs, creating Bi nanoparticles embedded in an amorphous Na-Mo-O matrix. The Bi particles then reversibly alloy with Na forming cubic Na3Bi leading to a specific capacity close to 300 mAh g−1 for the 10 first cycles. This is followed by a rapid capacity decay where the sodiation of Bi becomes irreversible leaving several inactive Na3Bi particles. To the best of our knowledge, this is due to the observed crystal growth of the Bi particles accompanied by structural changes in the insulating Na-Mo-O leading to poor conductivity in the electrodes. The poor electronic conductivity of the matrix deactivates the NaxBi particles and prevents the formation of the solid electrolyte interface layer as shown by post-mortem scanning electron microscopy studies.

• Brennhagen, Anders; Cavallo, Carmen; Wragg, David Stephen; Ponniah, Vajeeston; Sjåstad, Anja Olafsen & Koposov, Alexey [Vis alle 7 forfattere av denne artikkelen] (2022). Tracing the (de)sodiation of Bi2MoO6 through good and bad times with operando XRD. Fulltekst i vitenarkiv Vis sammendrag

  Anode materials combining conversion and alloying mechanisms (CAMs) are promising for Na-ion batteries, but their complex cycling mechanisms are challenging to study1. Understanding the (de)sodiation mechanism is crucial and, in several cases, requires advanced synchrotron characterization. Operando synchrotron studies are generally limited to one or two sodiation cycles, which are not fully comprehensive for CAMs2. Therefore, herein, we studied the sodiation and desodiation of Bi2MoO6-based anodes with laboratory-based operando X-ray diffraction exceeding more than 30 cycles to have a complete overview of our CAM’s mechanism (Figure 1)3. This revealed important aspects of the cycling and degradation mechanisms in the material. During the first sodiation, an irreversible conversion of Bi2MoO6 occurs, creating Bi nanoparticles embedded in an amorphous Na-Mo-O matrix. The Bi particles then reversibly alloy with Na forming cubic Na3Bi leading to a specific capacity close to 300 mAh g−1 for the 10 first cycles. This is followed by a rapid capacity decay where the sodiation of Bi becomes irreversible leaving several inactive Na3Bi particles. To the best of our knowledge, this is due to the observed crystal growth of the Bi particles accompanied by structural changes in the insulating Na-Mo-O leading to poor conductivity in the electrodes. The poor electronic conductivity of the matrix deactivates the NaxBi particles and prevents the formation of the solid electrolyte interface layer as shown by post-mortem scanning electron microscopy studies.

• Brennhagen, Anders; Cavallo, Carmen; Wragg, David Stephen; Ponniah, Vajeeston; Sjåstad, Anja Olafsen & Koposov, Alexey [Vis alle 7 forfattere av denne artikkelen] (2022). Corrigendum: Operando XRD studies on Bi2MoO6 as anode material for Na-ion batteries (Nanotechnology (2022)33 (185402) DOI: 10.1088/1361-6528/ac4eb5). Nanotechnology. ISSN 0957-4484. 33(35). doi: 10.1088/1361-6528/ac72b2. Fulltekst i vitenarkiv
• Brennhagen, Anders; Wragg, David; Fjellvåg, Helmer; Vajeeston, Ponniah; Sjåstad, Anja Olafsen & Cavallo, Carmen (2021). Beyond Insertion: Conversion alloying Bi/RGO-based materials as novel anodes for Na-ion batteries. Fulltekst i vitenarkiv
• Hessevik, Julie; Fjellvåg, Asbjørn Slagtern; Iveland, Oskar; By, Thomas; Skjelstad, Johan & Waller, David [Vis alle 8 forfattere av denne artikkelen] (2021). LaNiO3 as a Pt catchment material in the ammonia oxidation process. Fulltekst i vitenarkiv
• Brennhagen, Anders; Cavallo, Carmen; Wragg, David Stephen; Sjåstad, Anja Olafsen; Ponniah, Vajeeston & Fjellvåg, Helmer (2021). Operando XRD studies on Bi2MoO6 as anode material for Na-ion batteries. Fulltekst i vitenarkiv Vis sammendrag

  Na-ion batteries (NIBs) could be a good alternative to Li-ion batteries (LIBs) due to the large abundance and availability of sodium resources. The search for good anode materials is one of the big challenges since graphite, which is the most common anode material for LIB, does not work well in NIBs. We are developing Bi2MoO6 as an anode material for NIBs, and will explain the main cycling mechanism. By utilizing a combination of conversion and alloying reactions, the material achieves high capacity while still maintaining a decent cycling stability. The complex cycling mechanism also makes it an interesting and challenging material to characterize during cycling. In this poster, we present high quality operando XRD data of the material during several cycles, acquired in the home lab. The data clearly shows that the reversible alloying reaction of Bi-metal to cubic Na3Bi gives the main capacity contribution, after the initial conversion reaction. This indicates formation of nanocrystalline Bi-particles, as nanocrystalline Bi-metal has shown the same reaction mechanism while microcrystalline Bi instead forms the hexagonal phase of Na3Bi. The molybdenum goes into an amorphous matrix that is only partially electrochemically active. The phases formed with molybdenum during cycling are difficult to characterize, due to their amorphous nature, and remain an unsolved mystery.

• Hessevik, Julie; Fjellvåg, Asbjørn Slagtern; Iveland, Oskar; By, Thomas; Skjelstad, Johan & Waller, David [Vis alle 8 forfattere av denne artikkelen] (2021). LaNiO3 as a Pt catchment material in the ammonia oxidation process. Fulltekst i vitenarkiv
• Brennhagen, Anders; Cavallo, Carmen; Wragg, David Stephen; Koposov, Alexey; Ponniah, Vajeeston & Sjåstad, Anja Olafsen [Vis alle 7 forfattere av denne artikkelen] (2021). Operando XRD study on Bi2MoO6 as anode material for Na-ion batteries. Fulltekst i vitenarkiv Vis sammendrag

  Na-ion batteries (NIBs) could be a good alternative to Li-ion batteries (LIBs) due to the large abundance and availability of sodium resources. The search for good anode materials is one of the big challenges since graphite, which is the most common anode material for LIB, does not work well in NIBs. We are developing Bi2MoO6 as an anode material for NIBs, and will explain the main cycling mechanism. By utilizing a combination of conversion and alloying reactions, the material achieves high capacity while still maintaining a decent cycling stability. The complex cycling mechanism also makes it an interesting and challenging material to characterize during cycling. In this poster, we present high quality operando XRD data of the material during several cycles, acquired in the home lab. The data clearly shows that the reversible alloying reaction of Bi-metal to cubic Na3Bi gives the main capacity contribution, after the initial conversion reaction. This indicates formation of nanocrystalline Bi-particles, as nanocrystalline Bi-metal has shown the same reaction mechanism while microcrystalline Bi instead forms the hexagonal phase of Na3Bi. The molybdenum goes into an amorphous matrix that is only partially electrochemically active. The phases formed with molybdenum during cycling are difficult to characterize, due to their amorphous nature, and remain an unsolved mystery.

• Brennhagen, Anders; Cavallo, Carmen; Wragg, David; Vajeeston, Ponniah; Sjåstad, Anja Olafsen & Fjellvåg, Helmer (2021). Bi4V2O11/RGO composite as conversion and alloying anode for Na-ion batteries. Fulltekst i vitenarkiv
• Hval, Halvor Høen; Tripathi, Alok Mani; Forseth, Sissel & Fjellvåg, Helmer (2020). Understanding the details of crystal structure and electronic states in LNMO. Fulltekst i vitenarkiv
• Hempel, Frida Sveen; Arstad, Bjørnar & Fjellvåg, Helmer (2020). Layered hexagonal materials - Differences in ionic conductivity for solid-state electrolyte. Fulltekst i vitenarkiv
• Cavallo, Carmen; Brennhagen, Anders; Wragg, David & Fjellvåg, Helmer (2020). Beyond Insertion: Conversion alloying Bi/RGO-based materials as anode for Na-ion batteries. Fulltekst i vitenarkiv
• Wragg, David; Fjellvåg, Øystein; Fjellvåg, Helmer; Sottmann, Jonas; Vaughan, Gavin B M & Michel, Marco Di [Vis alle 7 forfattere av denne artikkelen] (2020). Operando Total Scattering Computed Tomography: Making the Invisible Visible. Fulltekst i vitenarkiv
• Fjellvåg, Øystein; Sønsteby, Henrik Hovde; Nilsen, Ola; Wolff, Max; Kapaklis, Vassilios & Fjellvåg, Helmer [Vis alle 8 forfattere av denne artikkelen] (2020). Towards Multiferroic Thin Film Composites by Atomic Layer Deposition (ALD). Fulltekst i vitenarkiv
• Wragg, David; Ruud, Amund; Fjellvåg, Øystein; Fjellvåg, Helmer; lebedev, Oleg & Michel, Marco Di [Vis alle 9 forfattere av denne artikkelen] (2019). PDF Analysis of Batteries Using Diffraction Computed Tomography. Fulltekst i vitenarkiv
• Wragg, David; Ruud, Amund; Fjellvåg, Helmer; Fjellvåg, Øystein; Vajeeston, Ponniah & Sottmann, Jonas [Vis alle 9 forfattere av denne artikkelen] (2019). PDF Analysis of Batteries Using Diffraction Computed Tomography. Fulltekst i vitenarkiv
• Wragg, David; Fjellvåg, Øystein; Ruud, Amund; Fjellvåg, Helmer; Vajeeston, Ponniah & Vaughan, Gavin B M [Vis alle 9 forfattere av denne artikkelen] (2019). XRD/PDF computed tomography and how to get the best possible operando data from ion batteries. Fulltekst i vitenarkiv
• Hval, Halvor Høen; Ruud, Amund; Forseth, Sissel; Emerich, Hermann; Nilsen, Ola & Fjellvåg, Helmer (2019). Operando investigation of batteries. Fulltekst i vitenarkiv
• Hval, Halvor Høen; Tripathi, Alok Mani & Fjellvåg, Helmer (2019). High Energy Density Electrodes for Lithium-ion Batteries and Operando Studies. Fulltekst i vitenarkiv
• Hval, Halvor Høen; Tripathi, Alok Mani; Ruud, Amund; Emerich, Hermann; Forseth, Sissel & Nilsen, Ola [Vis alle 7 forfattere av denne artikkelen] (2019). Make Batteries Safe Again. Fulltekst i vitenarkiv
• Hval, Halvor Høen; Tripathi, Alok Mani; Ruud, Amund; Emerich, Hermann; Forseth, Sissel & Nilsen, Ola [Vis alle 7 forfattere av denne artikkelen] (2019). Make Batteries Safe Again. Fulltekst i vitenarkiv
• Brennhagen, Anders; Cavallo, Carmen; Wragg, David Stephen; Ponniah, Vajeeston; Sjåstad, Anja Olafsen & Fjellvåg, Helmer (2024). Bi metallates as conversion-alloying anodes for Na-ion batteries. Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  Rechargeable batteries are essential for the green energy transition and Li-ion batteries (LIBs) are currently dominating the marked. Li and other elements used in LIBs are limited resources on the planet, which makes them relatively expensive. Na-ion batteries (NIBs) work similarly to LIBs, but with slightly lower capacities due to the higher mass of Na compared to Li. However, the much higher and widespread abundance of Na could make NIBs cheaper and more sustainable that LIBs, perfect for large scale stationary energy storage. One of the major challenges with NIBs is to find a good anode material. In this work, we have not been able to do that. However, we have shown how a group of anode materials works during charge and discharge to a much greater detail than before, and understood why they do not perform so well. We used these results to improve the performance of the materials, but not enough to be of commercial interest. Nevertheless, the methods, ideas and knowledge developed in this thesis can be used for other materials and lead to a great improvements of NIBs in the future.

• Amedi, Salah Bra & Fjellvåg, Helmer (2023). Syntese av lavvalent kompleksfluorider. Universitetet i Oslo. Fulltekst i vitenarkiv
• Hempel, Frida Sveen & Fjellvåg, Helmer (2023). Substituted Na-based Layered Tellurates: Synthesis, Local Structure and Na-Dynamics. Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  Solid-state batteries are currently limited by low ionic conductivity compared to the liquid analogue, and improving ionic conductivity is a major concern for commercialization. This thesis focuses on the local structure and ionic conductivity mechanism of a solid-state electrolyte candidate (the layered oxide Na2Zn2TeO6). The Na content of layered materials is well known to influence conductivity and phase stability, and in this work this is changed by substituting the framework cations Zn and Te with Ga and Sb. The first work of this thesis establishes a reliable synthesis route for the substituted materials, ensuring and confirming homogeneous distribution of the substituent by nuclear magnetic resonance spectroscopy (NMR). The decreased Na content is found to increase local motion of Na, but this is not translated to long range motion of ionic conductivity. Decreasing the Na-content is therefore not enough to improve ionic conductivity. This work finally made an in-depth study of complex ordering schemes that may occur in multicomponent layered oxides. The real situation can indeed be very complex, compared to the rather simple average picture description.

• Raniseth, Amund Midtgard; Hval, Halvor Høen; Fjellvåg, Helmer & Samarasingha, Pushpaka (2022). Synthesis, structural characterization, electrochemical properties, and thermal stability of Al-substituted LiMn1.5Ni0.5O4. Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  One of the interesting lithium-ion battery cathode materials which have been developed over the last 20 years is the lithium manganese nickel oxide (LMNO). However, problems with lifetime remain. Increasing the lifetime of LMNO is crucial to its commercial adoption, and one of the promising techniques to do this is substituting different elements of the structure. In this thesis, the structure, electrochemistry, and thermal stability of LMNO have been investigated with different amounts of aluminum substitution attempted to replace either manganese, nickel, or both elements evenly. The findings suggest that substituting manganese is not easily done, as impurities form. However, by replacing mainly nickel, aluminum is successfully incorporated into the structure at a stoichiometric amount of 0.2. This type of substitution was also found to change the electrochemical voltage plateau of the material, in addition to prolonging its cycle life. The aluminum substituted material also exhibited better thermal stability above 100℃.

• Skautvedt, Casper; Fjellvåg, Helmer; Koposov, Alexey; Brennhagen, Anders; Cavallo, Carmen & Sjåstad, Anja Olafsen (2022). Syntese og karakterisering av BiFeO3 som anodemateriale i Na-ionebatterier. Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  Na-ion batteries (NIBs) are currently an immature technology but have gained interest in research in the last years. A big field within NIBs is on the anode since there is none good enough yet. In this work, we have looked at BiFeO3 as an anode material for NIBs. Bi-based anodes have been investigated in the group before this work and have shown promising results. The choice of BiFeO3 was based on the interesting mechanism that occurs during charging and discharging when used as an anode. The mechanism is a conversion-alloying reaction and combines conversion and alloying reactions when BiFeO3 is sodiated. We have investigated this mechanism in detail with combined X-ray diffraction (XRD) and galvanostatic cycling (GS) with operando-XRD in the RECX-lab at the Department of Chemistry. BiFeO3 was synthesized by ceramic solid-state synthesis and sol-gel synthesis as part of the work. It has previously been synthesised with these syntheses but has proven to be difficult to be phase pure. The product of the syntheses has been characterized with XRD and refined with the Rietveld method. The ceramic solid-state synthesis gave a low yield of BiFeO3 and several impurities, but the sol-gel synthesis gave a high yield of BiFeO3 and few impurities. Various treatments of BiFeO3 were investigated, including ball milling, leaching and doping, to provide increased performance as a battery material. These tests showed that ball milling of the material gave the best results, but it was still not possible to find an optimisation that avoided a large drop in capacity during the first 30 cycles. To characterise BiFeO3 as an anode material, we test it in half-cells against Na metal. These half-cells were electrochemically analyzed by GS and cyclic voltammetry (SV). We also tested different mixtures of electrode composition and electrolytes to optimise battery properties. These showed that NaPF6 was the best salt and propylene carbonate (PC) + 5 wt% fluoroethylene carbonate (FEC) gave the most stable battery cells. The data from the electrochemical characterization and operando-XRD were used together to interpret the reaction mechanisms of BiFeO3. We saw from these that there were similarities in the alloying reaction between Na and Bi with other Bi-based anodes, with NaBi and Na3Bi as different stages in the reaction. However, there were no indications of the phase(s) of Fe after the reaction.

• Schuler, Raphael; Fjellvåg, Helmer & Norby, Truls Eivind (2021). Oxide thermoelectrics - materials, junctions and modules. Matematisk Naturvitenskapelig fakultet, Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  Thermoelectric materials are able to directly convert heat into electricity, which makes them predestined candidates for the recuperation of waste heat and contributing to a more efficient energy economy. They hold enormous potential for energy generation, since all thermodynamic processes lose a major portion of their energy in the form of waste heat. However, conventional thermoelectric materials are often toxic, scarce and/or expensive, and show limited high temperature stability, which currently limits their relevance for large-scale applications. Recently, oxide thermoelectric materials, composed of abundant elements have attracted increased interest, as they offer a cheaper, environmental-friendly, and high-temperature stable alternative. They are usually discovered and researched individually, and only later combined to a p-n pair in a module, which often leads to complications, like thermal mismatch or reactions. In this work the approach was taken to design a compatible pair of new thermoelectric oxides that preclude complications in advance. This novel approach is demonstrated on a model pair of iron tungstates, which show promising thermoelectric properties and an exceptional band alignment, which could prove to be key properties for future thermoelectrics.

• Perizat, Berdiyeva; Hauback, Bjørn Christian; Deledda, Stefano & Fjellvåg, Helmer (2020). Characterization of Heat Storage Materials with Neutron Scattering and Imaging Techniques. Matematisk Naturvitenskapelig fakultet, Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  Roughly, three-quarters of the energy produced in the world ends up as waste heat. However, the waste heat can be harvested, stored and reused using specially built heat storage reactors for high- or low-temperature applications. This thesis attempts to provide new insight into metal halide ammines as a potential heat storage system with a high energy efficiency. These materials undergo desorption and absorption of NH3-gas in a thermochemical reaction, which results in an uptake or release of heat, respectively. This work shows the study of the thermochemical heat storage reactor prototype with the metal halide ammine salt Sr(NH3)xCl2 for low-temperature applications below 100 °C. The changes in kinetics and chemical- and physical properties during NH3 cycling were characterized by neutron imaging and scattering techniques. A honeycomb-shaped heat exchanger within the reactor was optimized based on the results from the neutron imaging investigations. The mechanical stresses and heat transfer within the heat exchanger were studied in detail. Overall, this project demonstrates for the first time the possibility of applying neutron imaging and scattering techniques for analysis of thermochemical heat storage materials and systems.

• Nygård, Magnus Moe; Hauback, Bjørn Christian; Sørby, Magnus Helgerud & Fjellvåg, Helmer (2020). Local and average structure in high-entropy alloys and associated metal deuterides. Matematisk Naturvitenskapelig fakultet, Universitetet i Oslo. Fulltekst i vitenarkiv
• Li, Xinyu; Fjellvåg, Helmer & Nilsen, Ola (2020). Na-Based Layered Tellurate Na2M2TeO6 (M = Zn, Mg): Synthesis, Crystal Structure and Na+ Ion Diffusion. Universitetet i Oslo. Fulltekst i vitenarkiv
• Skaar, Erik; Fjellvåg, Helmer & Sønsteby, Henrik Hovde (2019). Structure-Property Relations in Thin Films of LaNiO3 and LaNi1-xCuxO3. Universitetet i Oslo. Fulltekst i vitenarkiv

Se alle arbeider i NVA