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• Lunde, Vilde G. S.; Fjellvåg, Øystein S.; Döring, Allan M.; Straßheim, Marc; Pomjakushin, Vladimir & Skokov, Konstantin P. [Vis alle 12 forfattere av denne artikkelen] (2026). Influence of Ni substitution on phase transitions and magnetocaloric effect of NdCo2 at cryogenic temperatures. Physical review B (PRB). ISSN 2469-9950. 113(18). doi: 10.1103/drsr-yt3j. Fulltekst i vitenarkiv
• Hemmatzade, A.; Balz, C.; Ollivier, J.; Lhotel, E.; Colonna, N. & Rufino, A. dos Santos [Vis alle 16 forfattere av denne artikkelen] (2026). Algebraic Loop Liquid in the Pyrochlore CsNiCrF6. Physical Review Letters. ISSN 0031-9007. 136(10). doi: 10.1103/xvxt-whns. Fulltekst i vitenarkiv Vis sammendrag

  The β -pyrochlore CsNiCrF 6 is a charge ice—each tetrahedron of the pyrochlore lattice is occupied by two Ni 2 + and two Cr 3 + according to an ice rule, resulting in fully packed same-species loops with power-law length distribution. Using inelastic neutron scattering and numerical simulations, we show that the spin dynamics can be well described by a model with three antiferromagnetic interaction strengths, one for Ni–Ni, Cr–Ni, and Cr–Cr interactions. Because the strengths of the two intraloop interactions are quite different, the fluctuations of each type of loop can be identified in the spectrum. CsNiCrF 6 therefore forms an algebraic loop liquid in which power-law cation correlations control the spin dynamics. Our parameters predict a transition to spin nematic order with associated arrest of dynamics at T n ≈ 0.55     K , while we observe a freezing transition at T f = 2.3     K . Measurements of magnetization, ac, and nonlinear susceptibility show that this is not a canonical spin glass transition so we associate it with the development of slow dynamics due to interloop nematic correlations.

• Lunde, Vilde G. S.; Ofstad, Benedicte; Ohresser, Philippe; Fjellvåg, Øystein Slagtern; Sjåstad, Anja O. & Hauback, Bjørn C. [Vis alle 7 forfattere av denne artikkelen] (2025). Electronic and magnetic properties of light rare-earth cubic Laves compounds derived from x-ray magnetic circular dichroism. Physical review B (PRB). ISSN 2469-9950. 112(22). doi: 10.1103/hf2c-46t3. Fulltekst i vitenarkiv Vis sammendrag

  This work presents electronic and magnetic properties of selected members in the cubic Laves phase series Nd1−𝑥⁢Pr𝑥⁡CoNi (0 ≤ 𝑥 ≤ 1) and Ce0.25⁢Pr0.75⁡CoNi, together with the corresponding binary compositions (NdCo2, NdNi2, PrCo2, PrNi2, CeCo2, CeNi2), using soft x-ray absorption spectroscopy, x-ray magnetic circular dichroism (XMCD), density-functional theory, and crystal field multiplet calculations. All transition-metal moments saturate below 1 T, while the rare-earth moments do not saturate even at 5 T, consistent with van Vleck paramagnetic contributions and crystal field suppression. While the sum rules are widely used to extract element-specific magnetic moments from XMCD, we show that for 3⁢𝑑 transition metals, their application requires accurate estimates of the number of unoccupied 3⁢𝑑 states. We observe a finite magnetic moment on Ni, challenging the common assumption of its nonmagnetic character in Laves phases. The orbital magnetic moments were determined using the spin rules, while the spin moments were estimated from single-ion values from multiplet calculations, due to the invalidity of the spin sum rule for light rare-earth elements. The magnetic moments of Nd and Pr are found to be suppressed relative to their free-ion values, with multiplet theory indicating that this is due to crystal field effects. Our results confirm that Nd and Pr maintain localized 4⁢𝑓3 and 4⁢𝑓2 configurations, respectively, and that their element-specific magnetic moments are robust to rare-earth substitution. Ce, on the other hand, exhibits a tuneable mixed-valent ground state with both magnetic 4⁢𝑓1 and nonmagnetic 4⁢𝑓0 components. The relative fraction of these states varies with the electronegativity of the surrounding 3⁢𝑑 transition metals, revealing a pathway to tune Ce magnetism via composition. This work establishes a framework for accurately interpreting XMCD in light rare-earth-based intermetallics and provides insight for designing light rare-earth-based magnetocaloric materials.

• Kim, Stephen; Sugai, Cody; Hauback, Bjørn Christian; Fjellvåg, Øystein Slagtern; Bradley, John P. & Ishii, Hope A. [Vis alle 8 forfattere av denne artikkelen] (2025). Enhanced hydrogenation kinetics of magnesium diboride through carbon substitution into the boride layers. Journal of Materials Chemistry A. ISSN 2050-7488. 13(48), s. 41896–41902. doi: 10.1039/d5ta07495a. Fulltekst i vitenarkiv
• Nikitin, S. E.; Andriushin, N. D.; Fjellvåg, Ø. S.; Pomjakushina, E.; Turrini, A. A. & Artyukhin, S. [Vis alle 8 forfattere av denne artikkelen] (2025). Competition between multiferroic and magnetic soliton lattice states in DyFeO3. Physical Review Research (PRResearch). 7(4). doi: 10.1103/kwrl-x4hw. Fulltekst i vitenarkiv Vis sammendrag

  Simultaneous breaking of time reversal and inversion symmetries in multiferroics couples ferroelectricity to magnetism and is a source of unusual physical phenomena that can be used in next-generation electronic devices. A notable example is DyFeO 3 , which under applied magnetic fields exhibits a giant linear magnetoelectric response and a large spontaneous electric polarization induced by coexisting orders of Fe and Dy subsystems. Here, we use high-resolution neutron diffraction to show that at zero field DyFeO 3 hosts an incommensurate magnetic soliton lattice formed by spatially ordered Dy domain walls with an average domain size of 231(8) Å. The long-ranged interaction between the domain walls is mediated by magnons propagating through the Fe subsystem and is analogous to the Yukawa force in particle physics. An applied magnetic field destroys the long-ranged incommensurate order, unlocks the linear magnetoelectric response, and stabilizes the ferroelectric state. The magnetic domain walls are electrically charged and the soliton array dimerizes when both electric and magnetic fields are applied. Numerical simulations with experimental parameters suggest that the generic competition between the ferroelectric and incommensurate states can be effectively controlled by an applied electric field.

• 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.

• Silva, Valmir Rodrigo da; Fjellvåg, Øystein Slagtern; Švec, Peter; Jr., Peter Švec,; Hauback, Bjørn Christian & Deledda, Stefano (2025). The Effect of Mo and Al Substitution in Cryomilled and Cold-Rolled FeNi Alloys. Metals. 15(9), s. 996–996. doi: 10.3390/met15090996. Fulltekst i vitenarkiv Vis sammendrag

  The ordered tetragonal FeNi L10 phase, tetrataenite, is a promising candidate for rare earth-free permanent magnets due to its competitive magnetic properties and the low cost of the constituent elements. In this work, we have investigated the effect of molybdenum and aluminum substitution on the formation of the ordered L10 phase. The alloys were prepared with die casting and melt spinning techniques, further processed using cold rolling and cryomilling, and finally annealed below the estimated order–disorder temperature (TOD). To study the influence of composition and processing of the alloys, structural characterization and microstructural analysis were performed with synchrotron radiation X-ray diffractometry (SR-PXD) and Scanning Transmission Electron Microscopy (STEM), respectively. The presence of tetrataenite in the alloys investigated in this work could not be confirmed. In situ SR-PXD and STEM indicated minimal structural changes in the temperature stability range of the materials. A full-loop hysteresis curve acquired using a vibrating sample magnetometer (VSM) indicated no signs of magnetic hardening of the alloys with the measured coercivity being below 10 Oe, and thus consistent with FeNi without ordering.

• 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.

• Boraley, X.; Stockert, O.; Lass, J.; Sibille, R.; Fjellvåg, Øystein Slagtern & Moody, S. H. [Vis alle 9 forfattere av denne artikkelen] (2025). Microscopic Origin of Reduced Magnetic Order in a Frustrated Metal. Physical Review Letters. ISSN 0031-9007. 135. doi: 10.1103/38ds-xjl3. Fulltekst i vitenarkiv
• 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.

• Yao, Weiliang; Liu, Supeng; Kikuchi, Hodaka; Ishikawa, Hajime; Fjellvåg, Øystein Slagtern & Tam, David W. [Vis alle 22 forfattere av denne artikkelen] (2025). Anomalous Electrical Transport in the Kagome Magnet YbFe6Ge6. Physical Review Letters. ISSN 0031-9007. 134(18). doi: 10.1103/PhysRevLett.134.186501. Fulltekst i vitenarkiv
• Andriushin, N. D.; Nikitin, S. E.; Fjellvåg, Øystein Slagtern; White, J. S.; Podlesnyak, A. & Inosov, D. S. [Vis alle 10 forfattere av denne artikkelen] (2025). Observation of the spiral spin liquid in a triangular-lattice material. Nature Communications. 16(1). doi: 10.1038/s41467-025-57319-2. Fulltekst i vitenarkiv
• Lass, Jakob; Moody, Samuel Harrison & Fjellvåg, Øystein Slagtern (2025). DMCpy: A powder and single crystal neutron diffraction software for the DMC diffractometer. SoftwareX. 31. doi: 10.1016/j.softx.2025.102237. Fulltekst i vitenarkiv
• Eggert, Bruno Guilherme Fischer; Delczeg-Czirjak, E.K.; Fjellvåg, Øystein Slagtern; Hauback, Bjørn Christian & Frommen, Christoph (2024). Structure and magnetism of Fe-substituted MnNiSi<inf>0.95</inf>Al<inf>0.05</inf>. Materialia. 39. doi: 10.1016/j.mtla.2024.102321. Fulltekst i vitenarkiv
• 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.

• 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.

• Gauthier, Nicolas; Sibille, Romain; Pomjakushin, Vladimir; Fjellvåg, Øystein Slagtern; Fraser, James & Desmarais, Mathieu [Vis alle 8 forfattere av denne artikkelen] (2024). Magnetic structure of Ce3TiBi5 and its relation to current-induced magnetization. Physical review B (PRB). ISSN 2469-9950. 109. doi: 10.1103/PhysRevB.109.L140405. Fulltekst i vitenarkiv
• Silva, Valmir Rodrigo da; Fjellvåg, Øystein Slagtern; Pokle, Anuj; Hauback, Bjørn Christian & Deledda, Stefano (2024). Effect of Carbon Addition and Mechanical Activation on FeNi Alloys for Permanent Magnet Applications. Metals. 14(10). doi: 10.3390/met14101125. Fulltekst i vitenarkiv Vis sammendrag

  Tetrataenite is a promising candidate for rare earth-free permanent magnets due to its low cost and intrinsic magnetic properties. This work investigates the effect of combined milling at liquid nitrogen temperatures (cryomilling) and the addition of carbon as an interstitial element for promoting the formation of tetrataenite. Crystal structure, microstructure, and magnetic properties are investigated to understand the influence of mechanical processing and compositional modifications. No unambiguous evidence of the ordered phase of tetrataenite is found in the structural characterization. However, using Scanning Transmission Electron Microscopy (STEM) and powder X-ray diffraction (PXD) analyses, the occurrence of both twinning and stacking faults resulting from the high-energy milling process is observed, which is a relevant factor for identifying tetrataenite in FeNi alloys. The probability of a stacking fault and twinning occurring for a carbon-free FeNi sample before annealing is found to be 2% and 1.4%, respectively. After annealing, the stacking fault probability decreased to 1.2%, while that of twinning was 1.4%. By increasing the carbon concentration to 5 at.%, the stacking faults and twinning probabilities decrease slightly to 1.2% and 1.3%, respectively. The occurrence of stacking faults combined with small crystallite sizes was a hindering factor in identifying the presence of tetrataenite.

• Chakravarty, Shruti; Fjellvåg, Øystein Slagtern; Bhattacharyya, Arpan; Keller, Lukas & Nair, Sunil (2023). Disorder-driven cluster glass state in a geometrically frustrated hexagonal perovskite. Physical review B (PRB). ISSN 2469-9950. 107(13), s. 1–12. doi: 10.1103/PhysRevB.107.134414. Fulltekst i vitenarkiv
• Sannes, Johnny Andreas; Gonano, Bruno; Fjellvåg, Øystein Slagtern; Kumar, Susmit; Nilsen, Ola & Valldor, Björn Martin (2023). X-ray and Neutron Diffraction Studies of SrTe<inf>2</inf>FeO<inf>6</inf>Cl, an Oxide Chloride with Rare Anion Ordering. Inorganic Chemistry. ISSN 0020-1669. 62(32), s. 13081–13088. doi: 10.1021/acs.inorgchem.3c01951. Fulltekst i vitenarkiv Vis sammendrag

  The oxychloride SrTe2FeO6Cl is obtained by high-temperature solid-state synthesis under inert conditions in closed reaction vessels. The compound crystallizes in a novel monoclinic crystal structure that is described in the space group P121/n1 (No. 14). The unit cell parameters, a = 10.2604(1) Å, b = 5.34556(5) Å, c = 26.6851(3) Å, and β = 93.6853(4)°, and atomic parameters were determined from synchrotron diffraction data, starting from a model that was obtained from single-crystal X-ray diffraction data. The anion lattice exhibits a rare ordering of oxide and chloride ions: one-dimensional zig-zag ladders of chlorine (squarelike motif) are surrounded by an oxygen matrix. Two different iron sites coordinated solely to oxygen are present in the structure, one octahedral and one square pyramidal, both distorted. Similarly, two different strontium coordinations are present; the first homoleptic coordinated to eight oxygen atoms and the second heteroleptic coordinated to four oxygen and four chlorine atoms in a fac-like manner. The lone pair of Te(IV) is directed toward the larger chlorine atoms. Magnetic susceptibility measurements confirm that Fe is +3 (d5) in the high-spin electronic configuration, exhibiting an almost ideal spin-only moment, μeff = 5.65 μB Fe–1. The slightly negative Weiss constant (θCW = −39 K) suggests dominating antiparallel spin-to-spin coupling in the paramagnetic temperature range, agreeing with an observed long-range antiferromagnetic spin ordering below Néel temperature, TN ∼ 13 K, and a broad second order-like anomaly in the specific heat measurement data. Low-temperature neutron diffraction data reveal that the antiferromagnetic ordered phase is C-type, with a k-vector (1/2, 1/2, 0) and ordered moment of 4.14(7) μB. The spin structure can be described as antiferromagnetic ordered layers stacked along the a-axis, forming layers of squares that alternate along the c-axis.

• Huangfu, Shangxiong; Austin, Alexandra C.; Guguchia, Zurab; Fjellvåg, Øystein Slagtern; Knorpp, Amy J. & Luetkens, H. [Vis alle 8 forfattere av denne artikkelen] (2023). Tuneable Short-Range Antiferromagnetic Correlation in Fe-Containing Entropy Stabilized Oxides. Inorganic Chemistry. ISSN 0020-1669. 63(1), s. 247–255. doi: 10.1021/acs.inorgchem.3c03028. Fulltekst i vitenarkiv
• 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.

• 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.

• Biswas, Banani; Michel, Veronica F.; Fjellvåg, Øystein; Bimashofer, Gesara; Döbeli, Max & Jambor, Michal [Vis alle 15 forfattere av denne artikkelen] (2022). Role of Dy on the magnetic properties of orthorhombic DyFeO3. PHYSICAL REVIEW MATERIALS. 6(7). doi: 10.1103/PhysRevMaterials.6.074401. Fulltekst i vitenarkiv
• 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.

• Ek, Gustav; Fjellvåg, Øystein; Vajeeston, Ponniah; Armstrong, Jeff; Sahlberg, Martin & Häussermann, Ulrich (2021). Vibrational properties of High Entropy Alloy based metal hydrides probed by inelastic neutron scattering. Journal of Alloys and Compounds. ISSN 0925-8388. 877, s. 1–7. doi: 10.1016/j.jallcom.2021.160320. Fulltekst i vitenarkiv Vis sammendrag

  The vibrational properties of several High Entropy Alloy (HEA) based metal hydrides are investigated by inelastic neutron scattering (INS). HEAs have recently emerged as a new type of materials with a wide range of intriguing properties and potential applications such as hydrogen storage. The special properties of HEAs are believed to originate from the disordered lattice and internal strain that is introduced from the differences in atomic radii. This makes HEA hydrides provide an intriguing situation for the local H coordination, of several different transition metals. INS spectra were collected on a series of HEA-based metal hydrides starting with TiVNbHx and subsequently adding Zr and Hf to increase the atomic size mismatch. A general feature of the spectra are the optical peaks centered around an energy loss of 150 meV that can be attributed to hydrogen vibrations in a tetrahedral environment. Upon the addition of Zr and Hf, a shoulder appears on the optical peak at lower energy transfers that after comparison with in silico calculated INS spectra is indicative of hydrogen also occupying octahedral sites in the structure.

• Fjellvåg, Asbjørn Slagtern; Fjellvåg, Øystein; Breard, Y. & Sjåstad, Anja Olafsen (2021). Structural disorder and antiferromagnetism in LaNi1–xPtxO3. Journal of Solid State Chemistry. ISSN 0022-4596. 229. doi: 10.1016/j.jssc.2021.122181. Fulltekst i vitenarkiv Vis sammendrag

  We report on the B-site substitution of Pt for Ni in the system LaNi1–xPtxO3. The system can only be synthesized between x ​≤ ​0.50, with LaNiO3 (x ​= ​0.00) and the stoichiometric double perovskite La2NiPtO6 (x ​= ​0.50) as the end members. Higher Pt-contents (x ​> ​0.50) are unachievable due to the preference of Pt to being in oxidation state ​+ ​IV in octahedral coordination. Upon introducing Pt into LaNiO3, a phase transformation from rhombohedral (R-3c) to monoclinic (P21/n) symmetry is observed for 0.075 ​≤ ​x ​≤ ​0.125, where all monoclinic samples are B-site ordered, and Pt show a strong preference for the Pt-site (2c-site) for all compositions (x ​≥ ​0.20). Powder X-ray diffraction analysis reveal disorder of the Pt-distribution in several of the non-equimolar samples (0.20 ​≤ ​x ​≤ ​0.40), which point toward cluster formation with domains of high and low Pt-content within each sample. La2NiPtO6 further show an antiferromagnetic transition at ∼ 40 ​K. A similar transition is observed for all monoclinic samples (x ​≥ ​0.20), however, the transition is less pronounced for lower x. This is explained in terms of the structural disorder, i.e. by the coexistence of antiferromagnetic domains with long range order and paramagnetic domains dominated by short range antiferromagnetic interactions.

• 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.

• Fjellvåg, Asbjørn Slagtern; Fjellvåg, Øystein; Kumar, Susmit; Ruud, Amund & Sjåstad, Anja Olafsen (2021). Interplay of valence states and magnetic interactions in the perovskite system LaNi1–xRhxO3. Journal of Solid State Chemistry. ISSN 0022-4596. 298. doi: 10.1016/j.jssc.2021.122124. Fulltekst i vitenarkiv Vis sammendrag

  We report on the structural, magnetic and electronic properties of Rh substituted LaNiO3, LaNi1–xRhxO3 (0 ​≤ ​x ​≤ ​1). Increased Rh contents are associated with increased structural distortion, and we observe a phase transformation and an immiscibility gap between 0.15 ​≤ ​x ​≤ ​0.25, separating the R-3c and Pnma perovskite structure regimes. Structural evaluation by synchrotron X-ray diffraction, X-ray absorption spectroscopy and DC magnetic measurements suggest variations in oxidation state for the B-site cations, with Ni(II)/Rh(IV), in addition to Rh(III), dominating for high Rh-contents. Although ordering of Ni(II)/Rh(IV) is possible, no B-site ordering between Ni and Rh is observed for any composition. A metal-to-insulator transition occurs upon Rh-substitution, correlating with the structural transition. DC- and AC magnetic measurements reveal that several of the compositions are magnetically frustrated with both antiferro- and ferromagnetic behaviour at low temperatures. The antiferromagnetic interactions are dominating, while the ferromagnetic interactions increase with increasing Rh-content, to a maximum at x ​= ​0.80 and x ​= ​0.90 in LaNi1–xRhxO3. The mixed valence states are assumed to contribute with a ferromagnetic super-exchange interaction between Ni(II) and Rh(IV).

• Nygård, Magnus Moe; Fjellvåg, Øystein; Sørby, Magnus Helgerud; Sakaki, Kouji; Ikeda, Kazutaka & Armstrong, Jeff [Vis alle 12 forfattere av denne artikkelen] (2021). The average and local structure of TiVCrNbDx (x = 0, 2.2, 8) from total scattering and neutron spectroscopy. Acta Materialia. ISSN 1359-6454. 205, s. 1–15. doi: 10.1016/j.actamat.2020.116496. Fulltekst i vitenarkiv Vis sammendrag

  The volumetric hydrogen density of 160 kg H/m 3 in TiVCrNbH 8 is among the highest for interstitial hy- drides, but the reported reversible capacity is only about 2/3 of the full theoretical capacity at room temperature. In the present work we have investigated the local structure in TiVCrNbD x , x = 0 , 2.2, 8 with the aim to unravel how the remaining sites can be destabilized with respect to hydrogen/deuterium occupation using total scattering measurements and Reverse Monte Carlo (RMC) structure modelling. Our analysis indicates that the partially desorbed deuteride ( x = 2 . 2 ) adopts a body-centred tetragonal struc- ture ( I4 /mmm ) where the deuterium atoms occupy both tetrahedral and octahedral interstices with low occupancies. There is a significantly higher portion of occupied sites with nearest-neighbour metals with low valence-electron concentration VEC . This observation is used to motivate strategies for further desta- bilization of the hydride. Inelastic neutron scattering (INS) and density functional theory (DFT) calcula- tions indicate that the vibrational density of states is very diverse in TiVCrNbH 2 . 4 , and it is suggested that the hydrogen atoms might be mobile between nearby interstices.

• 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.

• 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.

• 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.

• 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.

• 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.

• Fjellvåg, Øystein; Nygård, Kristin Hubred; Vajeeston, Ponniah & Sjåstad, Anja Olafsen (2019). Advances in the LiCl salt flux method and the preparation of phase pure La2−xNdxLiHO3 (0 ≤ x ≤ 2) oxyhydrides. Chemical Communications. ISSN 1359-7345. 55(26), s. 3817–3820. doi: 10.1039/c9cc00920e. Fulltekst i vitenarkiv Vis sammendrag

  The LiCl salt flux method is an established aid in oxyhydride synthesis. By operating the flux below its melting point, we have obtained phase pure Nd2LiHO3 for the first time. Further, the suitability of the flux method is shown to be dictated by a delicate balance between the thermal stability of the oxyhydride in question and the ionic mobility of the reactants.

• Fjellvåg, Øystein; Krzystyniak, Matthew; Vajeeston, Ponniah; Sjåstad, Anja Olafsen & Armstrong, Jeff (2019). A combined deep inelastic neutron scattering andab initiolattice dynamics study of the hydride anion dynamics and bonding in La2LiHO3 oxyhydride. Journal of Physics Communications. 3(10). doi: 10.1088/2399-6528/ab4be6. Fulltekst i vitenarkiv Vis sammendrag

  Kobayashi et al (6279) (Science 2016, 351) reported recently the existence of pure H- conductivity in the oxyhydride La2−x−y Sr x+y LiH1−x+y O3−y , while demonstrating its functionality through a prototype solid-state Ti/La2LiHO3/TiH2 battery. In this study, we probe the atomistic motion of La2LiHO3 obtained by the promising halide salt flux method, via a combination of deep inelastic neutron scattering (DINS) and ab initio lattice dynamics (LD) calculations verified by vibrational inelastic neutron spectroscopy (INS). We successfully describe the measured momentum distributions from DINS via our LD calculations, without observing any diffusion activation over the temperature range reported by Kobayashi et al. This observation is corroborated by model predictions from our LD study, which reveals that the hydride anions remain bound within a 3D-harmonic potential. We conclude that with the current synthesis parameters, the method produces a vacancy free lattice, and that a necessary ingredient for diffusive motion of H- is the presence of a large population of vacancies. Based on the harmonic prediction for the hydrogen kinetic energy, we derive a picture of the evolution of the effective bonding potential for the hydride anions, and link this to the dynamics associated with decomposition of the oxyhydride.

• Kumar, Susmit; Fjellvåg, Øystein; Sjåstad, Anja Olafsen & Fjellvåg, Helmer (2019). Physical properties of Ruddlesden-Popper (n = 3) nickelate: La4Ni3O10. Journal of Magnetism and Magnetic Materials. ISSN 0304-8853. 496. doi: 10.1016/j.jmmm.2019.165915. Fulltekst i vitenarkiv Vis sammendrag

  Nickelates, just like their isoelectronic cousins (the cuprate superconductors), are candidates for high temperature superconductivity. As charge density waves (CDW) and superconductivity might share a similar microscopic origin and that CDW behaviour is earlier suggested for La4Ni3O10, we here provide a detailed physical characterization of the magnetic, electronic and thermodynamic properties of stoichiometric polycrystalline La4Ni3O10 at and below 300 K. By variable temperature, synchrotron powder X-ray diffraction, we observe noticeable expansion in the b-axis and a contraction in c-axis coinciding with the electronic transition. Magnetic measurements indicate Pauli paramagnetic behaviour for the entire temperature range of 4–300 K. Transport data suggest electron–phonon interaction based conduction from 150 to 300 K, at 138 K the we observe an anomaly of second order nature, previously reported as a CDW anomaly. Below this anomaly, we observe no indications of electron–phonon interaction, but rather, Fermi liquid (FL) based T2 dependence with resistivity minima at 20 K, below which T1/2 dependence occurs for the resistivity indicating pure electron–electron interactions (EEI) based transport. Specific heat data indicates that mass enhancement of La4Ni3O10 similar to that of the normal state of the cuprates and the entropy change at anomaly suggests close similarities to other quasi-two dimensional systems, such as molybdenum bronzes and manganites.

• Liu, Xin; Bjørheim, Tor Svendsen; Vines, Lasse; Fjellvåg, Øystein; Granerød, Cecilie Skjold & Prytz, Øystein [Vis alle 10 forfattere av denne artikkelen] (2019). Highly Correlated Hydride Ion Tracer Diffusion in SrTiO3–xHx Oxyhydrides. Journal of the American Chemical Society. ISSN 0002-7863. 141(11), s. 4653–4659. doi: 10.1021/jacs.8b12985. Fulltekst i vitenarkiv Vis sammendrag

  Mixed oxide hydride anion systems constitute a novel class of materials exhibiting intriguing properties such as solid-state hydride ion conduction and fast anion exchange. In this contribution we derive the kinetics of hydride ion transport in a mixed oxide–hydride system, SrTiO3–xHx, through isotope exchange and depth profiling. Density functional theory (DFT) calculations indicate that migration of H– to neighboring vacant oxygen lattice sites is fast, but that long-range transport is impeded by slow reorganization of the oxygen sublattice. From measured hydride tracer-diffusion coefficients and the correlation factors derived from DFT, we are able to derive the hydrogen self-diffusion coefficients in SrTiO3–xHx. More generally, the explicit description of hydride ion transport in SrTiO3–xHx through combination of experimental and computational methods reported in this work can be applied to explore anion diffusion in other mixed anion systems.

• Fjellvåg, Øystein; Øygarden, Vegar; Sørby, Magnus Helgerud & Sjåstad, Anja Olafsen (2019). Crystal structure of LaSr3Fe3O9 and its phase relation with LaSr3Fe3O10. Journal of Solid State Chemistry. ISSN 0022-4596. 275, s. 56–62. doi: 10.1016/j.jssc.2019.03.050. Fulltekst i vitenarkiv Vis sammendrag

  The Ruddlesden-Popper phase LaSr3Fe3O10 and its reduced derivatives with nominal composition LaSr3Fe3O10–x, with 0 < x < 1.21, are described in detail with respect to crystal structure, magnetic order, thermal stability and likely vacancy mechanisms. The crystal structure of the LaSr3Fe3O9 (x = 1) phase is described. The LaSr3Fe3O10-δ phase prevails for x < 0.51, whereas LaSr3Fe3O9±ε prevails for x > 0.85. The phases are separated by a two-phase region 0.51 < x < 0.85. By combined Rietveld refinements of high-resolution powder synchrotron and neutron diffraction, the crystallographic and magnetic structure of LaSr3Fe3O9 is described. LaSr3Fe3O9 takes a layer like RP3 type structure with features of the brownmillerite structure in the triple perovskite type slabs; unit cell dimensions a = 28.7562(13) Å, b = 5.5280(2) Å, c = 5.4583(2) Å, space group Cmcm. LaSr3Fe3O9 is an antiferromagnet with TN > 350 K and with a G-type magnetic structure, magnetic space group PCbcm. Based on unit cell volume considerations, local oxygen vacancy ordering schemes are suggested.

• Fjellvåg, Øystein Slagtern; Armstrong, Jeff; Vajeeston, Ponniah & Sjåstad, Anja Olafsen (2018). New Insights into Hydride Bonding, Dynamics, and Migration in La2LiHO3 Oxyhydride. The Journal of Physical Chemistry Letters. s. 353–358. doi: 10.1021/acs.jpclett.7b03098. Fulltekst i vitenarkiv Vis sammendrag

  Hydride anion-conducting oxyhydrides have recently emerged as a brand new class of ionic conductors. Here we shed a first light onto their local vibrations, bonding mechanisms, and anion migration properties using the powerful combination of high-resolution inelastic neutron scattering and a set of rigorously experimentally validated density functional theory calculations. By means of charge-density analysis we establish the bonding to be strongly anisotropic; ionic in the perovskite layer and covalent in the rock salt layer. Climbing nudged elastic band calculations allow us to predict the hydride migration paths, which crucially we are able to link to the observed exotic ionic–covalent hybrid bonding nature. In particular, hydride migration in the rock salt layer is seen to be greatly hindered by the presence of covalent bonding, forcing in-plane hydride migration in the perovskite layer to be the dominant transport mechanism. On the basis of this microscopic insight into the transport and bonding, we are able to propose future candidates for materials that are likely to show enhanced hydride conductivity.

• Getz, Michael; Hansen, Per-Anders Stensby; Fjellvåg, Øystein Slagtern; Ahmed, Mohammed Amin Karim; Fjellvåg, Helmer & Nilsen, Ola (2017). Intense NIR emission in YVO4:Yb3+ thin films by atomic layer deposition. Journal of Materials Chemistry C. ISSN 2050-7526. 5(33), s. 8572–8578. doi: 10.1039/c7tc02135f. Fulltekst i vitenarkiv Vis sammendrag

  We demonstrate controlled deposition by atomic layer deposition of YVO4:Yb3+ thin films exhibiting an intense NIR emission under UV excitation after post-deposition annealing at 1000 °C. The samples are deposited using the precursor combinations Yb(thd)3, Y(thd)3, and VO(thd)2 with O3 (thd = 2,2,6,6-tetramethyl-3,5-heptanedione) at deposition temperatures between 260 and 300 °C. The NIR emission is expected to be due to a quantum splitting process in which one UV photon is converted to two NIR photons. In the present contribution we assess the efficiency of the UV to NIR conversion in the produced films and determine the optimal pulse parameters with respect to NIR emission intensity. We also demonstrate the synthesis of a crystalline YVO4:YbVO4 layered material by ALD to verify the possibility of creating artificial core–shell type structures. The structure, thickness, and composition of the deposited films have been studied by X-ray diffraction, ellipsometry, and X-ray fluorescence, respectively.

• Napari, M; Tarvainen, O; Kinnunen, S; Arstila, K; Julin, J & Fjellvåg, Øystein Slagtern [Vis alle 9 forfattere av denne artikkelen] (2017). The α and γ plasma modes in plasma-enhanced atomic layer deposition with O2-N2 capacitive discharges. Journal of Physics D: Applied Physics. ISSN 0022-3727. 50(9). doi: 10.1088/1361-6463/aa59b3. Fulltekst i vitenarkiv Vis sammendrag

  Two distinguishable plasma modes in the O2–N2 radio frequency capacitively coupled plasma (CCP) used in remote plasma-enhanced atomic layer deposition (PEALD) were observed. Optical emission spectroscopy and spectra interpretation with rate coefficient analysis of the relevant processes were used to connect the detected modes to the α and γ modes of the CCP discharge. To investigate the effect of the plasma modes on the PEALD film growth, ZnO and TiO2 films were deposited using both modes and compared to the films deposited using direct plasma. The growth rate, thickness uniformity, elemental composition, and crystallinity of the films were found to correlate with the deposition mode. In remote CCP operations the transition to the γ mode can result in a parasitic discharge leading to uncontrollable film growth and thus limit the operation parameters of the capacitive discharge in the PEALD applications. © 2017 IOP Publishing

• Fjellvåg, Øystein Slagtern; Armstrong, Jeff; Slawinski, Wojciech Andrzej & Sjåstad, Anja Olafsen (2017). Thermal and Structural Aspects of the Hydride-Conducting Oxyhydride La2LiHO3 Obtained via a Halide Flux Method. Inorganic Chemistry. ISSN 0020-1669. 56(18), s. 11123–11128. doi: 10.1021/acs.inorgchem.7b01409. Fulltekst i vitenarkiv Vis sammendrag

  Oxyhydrides, in which oxide and hydride anions share the same anionic lattice, are relatively rare compounds. La2LiHO3 belongs to this family. We report the synthesis of La2LiHO3 by means of an alkali halide flux method, which allows the production of larger quantities of material relative to the usually adopted synthesis routes. Powder X-ray and neutron diffraction studies show that La2LiHO3 adopts an n = 1 Ruddlesden–Popper (RP)-type structure with an orthorhombic distortion (Immm) due to hydride and oxide anion ordering. No sign of polymorphism is observed. La2LiHO3 is seen to decompose in an oxygen atmosphere at ∼450 °C into La2LiO3.5. We show that the high mobility of hydride anions close to the decomposition temperature is likely the main factor in inducing the oxidation. The crystal structure of La2LiO3.5 is also determined and takes an n = 1 RP-type structure with an orthorhombic distortion (Fmmm). This newly reported large-scale synthesis approach, combined with the proven high thermal stability, is a key factor for potential practical applications of this oxyhydride in real devices.

• Slawinski, Wojciech Andrzej; Fjellvåg, Øystein Slagtern; Ruud, Amund & Fjellvåg, Helmer (2016). A novel polytype - The stacking fault based γ-MoO3 nanobelts. Acta Crystallographica. Section B: Structural Science, Crystal Engineering and Materials. ISSN 2052-5192. 72(2), s. 201–208. doi: 10.1107/S2052520615024804. Fulltekst i vitenarkiv
• Gan, Jiantuo; Gorantla, Sandeep Madhukar; Riise, Heine Nygard; Fjellvåg, Øystein Slagtern; Diplas, Spyridon & Løvvik, Ole Martin [Vis alle 9 forfattere av denne artikkelen] (2016). Structural properties of Cu2O epitaxial films grown on c-axis single crystal ZnO by magnetron sputtering. Applied Physics Letters. ISSN 0003-6951. 108:152110(15), s. 1–5. doi: 10.1063/1.4945985. Fulltekst i vitenarkiv Vis sammendrag

  Epitaxial Cu2O films grown by reactive and ceramic radio frequency magnetron sputtering on single crystalline ZnO (0001) substrates are investigated. The films are grown on both O- and Zn-polar surface of the ZnO substrates. The Cu2O films exhibit a columnar growth manner apart from a ∼5 nm thick CuO interfacial layer. In comparison to the reactively sputtered Cu2O, the ceramic-sputtered films are less strained and appear to contain nanovoids. Irrespective of polarity, the Cu2O grown by reactive sputtering is observed to have (111)Cu2O||(0001)ZnO epitaxial relationship, but in the case of ceramic sputtering the films are found to show additional (110)Cu2O reflections when grown on O-polar surface. The observed CuO interfacial layer can be detrimental for the performance of Cu2O/ZnO heterojunction solar cells reported in the literature.

Se alle arbeider i NVA

• Kosler, Josef; Sjåstad, Anja Olafsen; Fjellvåg, Øystein Slagtern; Frommen, Christoph & Hauback, Bjørn Christian (2026). Study of Magnetic and Structural Properties of Ho1-xNdxVO3 for Hydrogen Liquefaction. Fulltekst i vitenarkiv
• Fjellvåg, Øystein Slagtern (2026). Spinon continuum of a triangular lattice Ising candidate. Fulltekst i vitenarkiv
• Lunde, Vilde Sturtzel; Fjellvåg, Øystein Slagtern; Pomjakushin, Vladimir; Sjåstad, Anja Olafsen; Hauback, Bjørn Christian & Frommen, Christoph (2025). Influence of Ni-Substitution on Structural and Magnetic Transitions of NdCo2. Fulltekst i vitenarkiv
• Fjellvåg, Øystein Slagtern (2025). A triangular lattice Ising model candidate. Fulltekst i vitenarkiv
• Fjellvåg, Øystein Slagtern (2025). Cr(II) fluoroperovskites – Jahn-Teller active materials. Fulltekst i vitenarkiv
• Fjellvåg, Øystein Slagtern (2025). Investigations of Cr(II) fluoroperovskites and other systems at SNBL . Fulltekst i vitenarkiv
• Lunde, Vilde Sturtzel; Fjellvåg, Øystein Slagtern; Frommen, Christoph; Sjåstad, Anja Olafsen & Hauback, Bjørn Christian (2025). Influence of Ni-Substitution on Structural and Magnetic Transitions of NdCo2. Fulltekst i vitenarkiv
• Fjellvåg, Øystein Slagtern (2024). Midlertidig ansettelse i akademia- muligheter og utfordringer. Fulltekst i vitenarkiv
• Fjellvåg, Øystein Slagtern (2024). A triangular lattice Ising model candidate. Fulltekst i vitenarkiv
• Fjellvåg, Øystein Slagtern; Hernandez, A.; Tam, D.; Roessli, B.; Shin, S. & Medarde, M. [Vis alle 9 forfattere av denne artikkelen] (2023). 2D Rare Earth Trihalides. Fulltekst i vitenarkiv
• Fjellvåg, Øystein Slagtern (2023). Frustrated magnetism in CeBr3. Fulltekst i vitenarkiv
• Fjellvåg, Øystein Slagtern; Keller, Lukas; Graf, D.; Zeitelhack, K.; Defendi, I. & Howard, A. [Vis alle 12 forfattere av denne artikkelen] (2023). Cold Neutron Diffractometer DMC at SINQ. Fulltekst i vitenarkiv
• Talib, F.; Fjellvåg, Øystein Slagtern & Kenzelmann, M. (2023). Diffuse magnetic scattering in rare earth halides. Fulltekst i vitenarkiv
• 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
• Sannes, Johnny Andreas; Gonano, Bruno; Fjellvåg, Øystein Slagtern; Kumar, Susmit; Nilsen, Ola & Valldor, Björn Martin (2023). X-ray and neutron diffraction studies of SrTe2FeO6Cl, an oxide chloride with rare ordering of anions. Fulltekst i vitenarkiv
• Fjellvåg, Øystein (2021). Neutron scattering to understand oxyhydrides, complex intermetallics, and fluoroperovskites. 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; Armstrong, J; Krzystyniak, Matthew; Vajeeston, Ponniah & Sjåstad, Anja Olafsen (2020). Structure and dynamics of La2LiHO3. 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
• Fjellvåg, Øystein; Hubred, Nygård Kristin; Ponniah, Vajeeston; Jeff, Armstrong; Matthew, Krzystyniak & Sjåstad, Olafsen Anja (2019). Synthesis, thermal stability and ionic mobility of the oxyhydride La2-xNdxLiHO3. Fulltekst i vitenarkiv
• Wragg, David; Sottmann, Jonas; Ruud, Amund; Fjellvåg, Helmer; Fjellvåg, Øystein & Vajeeston, Ponniah [Vis alle 9 forfattere av denne artikkelen] (2018). Operando X-ray Diffraction Studies of Battery Materials. Fulltekst i vitenarkiv
• Wragg, David; Sottmann, Jonas; Ruud, Amund; Fjellvåg, Øystein & Vaughan, Gavin B M (2018). Combined refinement of XRD and PDF data from nanomaterials. Fulltekst i vitenarkiv
• Wragg, David; Ruud, Amund; Sottmann, Jonas; Vajeeston, Ponniah; Fjellvåg, Helmer & Fjellvåg, Øystein [Vis alle 8 forfattere av denne artikkelen] (2018). New Anodes for Sodium and Lithium Ion Batteries. Fulltekst i vitenarkiv
• Fjellvåg, Øystein Slagtern & Sjåstad, Anja Olafsen (2017). Oxyhydrides, a new material class of complex mixed anion compounds. Fulltekst i vitenarkiv
• Fjellvåg, Øystein Slagtern; Sjåstad, Anja Olafsen & Armstrong, Jeff (2016). Hydride oxides with perovskite related structure: Case study of La2LiHO3. Fulltekst i vitenarkiv
• Fjellvåg, Øystein Slagtern; Sjåstad, Anja Olafsen; Haugsrud, Reidar & Gunnæs, Anette Eleonora (2015). Investigation of Novel Hydride Oxides for Holistic Understanding of Functional Materials. Fulltekst i vitenarkiv
• Fjellvåg, Øystein Slagtern; Fjellvåg, Helmer; Liu, Xin; Haugsrud, Reidar & Sjåstad, Anja Olafsen (2015). Hydride oxides with perovskite related structure: Case study of La2LiHO3. Fulltekst i vitenarkiv
• Fjellvåg, Øystein Slagtern; Ruud, Amund; Sønsteby, Henrik; Fjellvåg, Helmer & Nilsen, Ola (2014). Crystallization and electrochemical investigation of α- and β-MoO3 thin films deposited by Atomic Layer Deposition. Fulltekst i vitenarkiv
• Sønsteby, Henrik; Østreng, Erik; Fjellvåg, Øystein Slagtern; Nilsen, Ola & Fjellvåg, Helmer (2014). Sodium- and Potassium Containing Thin Films by Atomic Layer Deposition. Fulltekst i vitenarkiv
• Tollin, Hallvard Rise; Sjåstad, Anja Olafsen & Fjellvåg, Øystein (2021). Syntese og karakterisering av hydridoksider. Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  Denne masteroppgaven har fokus på syntese og karakterisering av hydridoksidene med sammensetningen Ln2LiHO3 (Ln = La, Nd), Sr substituert La2LiHO3, LaSr3Fe3HxOy og HoHO. Motivasjonen for valg av de nevnte hydridoksidsystemene er som følgende: Ln2LiHO3 (Ln = La, Nd) er velstudert og derfor en god kandidat for å få mekanistisk innsikt i hvordan hydridoksider nukleerer og gror i en saltfluks. Sr substituert La2LiHO3 har potensiale som et ioneledende materiale med mobilitet av hydridaniont, mens for HoHO er ikke trend for ordning av anioner forstått relativt til Ln-ioneradius (Ln &lt; Nd gir uordnet aniongitter) og dette må verifiseres. LaSr3Fe3HxOy er interessant som hydridoksid da det aldri tidligere er rapportert hydridoksid inneholdende det 3d redoks-aktive jern-kationet, eller som inntar RP3-struktur. Systemene Ln2LiHO3 (Ln = La, Nd) og Sr substituert La2LiHO3, ble syntetisert med fluksmetoden. Det ble også utviklet metodikk for in –situ pulver-røntgendiffraksjon studier av nukleering og vekst av Nd2LiHO3. LaSr3Fe3HxOy ble syntetisert ved topotaktisk ionebytte og HoHO med kerammetoden. Strukturanalyse ble gjort med røntgendiffraksjon, og i tillegg ble det foretatt termisk stabilitetsanalyser med TGADSC. Forbindelsene LaSr3Fe3HxOy og HoHO ble studert med nøytrondiffraksjon. For første gang er Sr substituert La2LiHO3 fremstilt med fluksmetoden. Srsubstitusjonsgraden er lavere enn forventet (ca. 20 % av nominell sammensetning), men det foreligger gode bevis for at en Sr-rik tetragonal fase stabiliseres i fluks ved høy konsentrasjon av Sr (x + y &gt; 0,5 i La2-x-ySrx+yLiH1+2yO3-(x/2)-y). For ortorombisk Sr substituert La2LiHO3 er substitusjonsmekanismen aliovalent anionsubstitusjon. HoHO tar en F-43m type struktur og er anion-ordnet med identisk bindingsavstand for Ho-O og Ho-H. Basert på TGA-DSC, strukturanalyse fra nøytrondata og BVS er hydridoksidet LaSr3Fe3H1,07O8,6 fremstilt. Det er allikevel usikkerhet knyttet til det faktum at forbindelsen er et hydridoksid, så dette må verifiseres.

• Fjellvåg, Øystein; Sjåstad, Anja Olafsen & Gunnæs, Anette Eleonora (2019). Oxyhydrides – Synthesis, Crystal Structure and Properties. Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  This thesis focus on the research of the new and scarcely explored material class oxyhydrides, where the anionic lattice consists of both oxide and hydride anions. Selected oxyhydrides described by the formula La2‑xNdxLiHO3 (0 ≤ x ≤ 2) have been obtained by a halide salt flux method in large quantities (gram scale). We describe key factors for preparation of oxyhydrides by this approach, and we discover lithium chloride to serve as an excellent flux also below its melting temperature, opening for a completely new temperature range for synthesis of oxyhydrides. Phase pure Nd2LiHO3 was in this work produced phase pure for the first time. Neutron diffraction was employed to study the long range and local structure of La2LiHO3, and neutron spectroscopy combined with computational methods yields new insight the migration of hydride anions in the compound based on evaluation of the chemical bonding. LaSr3Fe3O9 is found to be a new layered compound with a G-type antiferromagnetic structure which is immiscible with LaSr3Fe3O10‑δ. Preliminary neutron diffraction experiments on corresponding oxyhydrides prepared by topotactic anion-exchange with CaH2, indicate successful preparation of an oxyhydride with the composition LaSr3Fe3H1.1O8.9.

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