ponniahv

• Mahalaxmi, Paraman; Veerapandy, Vasu & Ponniah, Vajeeston (2026). First-Principles Investigation into Structural and Optoelectronic Properties of Mixed Halide Perovskites CsPbBr<inf>3–y</inf>X<inf>y</inf>(X = I, Cl, F; y = 0, 1, 2, 3) Using GGA, mBJ, mBJ + SOC, and HSE Functionals. ACS Omega. 11(37), s. 56209–56223. doi: 10.1021/acsomega.6c06754. Fulltekst i vitenarkiv
• Wanniarachchige, Chapa Pamodani Wanniarachchi; Ponniah, Vajeeston; Rahman, Talal & Velauthapillai, Dhayalan (2026). Mo- and Mo, X- doped (X = S, Se, Te) Anatase TiO2: insights from DFT and AIMD. Materials Research Express. 13(12). doi: 10.1088/2053-1591/ae79fc. Fulltekst i vitenarkiv Vis sammendrag

  Abstract Ab initio molecular dynamics (AIMD) simulations within the NVT ensemble using the Nosé thermostat along with density functional theory (DFT) calculations to investigate the structural, dynamic, thermodynamic, and electronic properties of a 108-atoms of Mo- and Mo, X-doped (X = S, Se, Te) anatase TiO 2 systems at 300, 450, and 600 K. The standard GGA + U bandgap of pure anatase TiO 2 (3.01 eV) is reduced to 0.99 eV (Mo), 1.52 eV (Mo, S), 1.68 eV (Mo, Se), and 1.43 eV (Mo, Te) due to the formation of mid-gap states and enhanced orbital hybridization, suggesting improved visible-light absorption. Our results show that Mo doping alone induces thermal and energetic instabilities, whereas co-doping with chalcogens (S, Se, and Te) progressively enhances lattice stability, with Mo, Te co-doping exhibiting thermal behaviour closest to that of pure TiO 2 . For the 48-atom models, optical property calculations reveal that Mo doping and Mo, X (X = S, Se, Te) co-doping strongly shift absorption from the ultraviolet into the visible and infrared regions. The co-doped systems exhibit enhanced absorption coefficients and lower reflectivity in the visible range, indicating superior light-harvesting capability, with the optical response red-shifting progressively as the anion size increases from S to Se to Te. AIMD simulations show that Mo-doped anatase, TiO 2 introduces local lattice rigidity but results in reduced thermal stability under finite-temperature conditions. Chalcogen co-doping (S, Se, and Te) enhances atomic mobility while improving overall thermal stability during AIMD simulations. Notably, Mo, Te- doped TiO 2 exhibits high thermal stability despite larger atomic displacements, indicating that stability is achieved through efficient energy dissipation via lattice flexibility. Radial distribution function analysis confirms that all systems retain a well-defined local Ti-O bonding environment, with no significant bond-length fluctuations or structural disorder, even in the presence of chalcogen co-doping.

• 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.

• Tharuman, Sharmila; Ponniah, Vajeeston; Chen, Shen Ming & Baskaran, Sambath (2025). Self powered real time sustainable energy harvesting and sensing using NiCo-Prussian blue analogue–based triboelectric nanogenerator. Chemical Engineering Journal. ISSN 1385-8947. 520. doi: 10.1016/j.cej.2025.165706. Fulltekst i vitenarkiv
• Balakrishnan, Kanimozhi; Veerapandy, Vasu; Nalini, Vajeeston & Ponniah, Vajeeston (2025). Density functional theory analysis of novel ZrO2 polymorphs: Unveiling structural stability, electronic structure, vibrational and optical properties. Computational Materials Science. ISSN 0927-0256. 246. doi: 10.1016/j.commatsci.2024.113439. Fulltekst i vitenarkiv Vis sammendrag

  The importance of advanced materials like zirconium dioxide (ZrO2) in diverse medical, industrial, and technological contexts is underscored by contemporary technology. ZrO2′s unique combination of properties renders it indispensable for a broad spectrum of applications, suggesting its enduring importance. This study presents the very first investigation into the physical properties, structural stability, and ground-state characteristics of sixteen distinct ZrO2 polymorphs through the application of density functional theory (DFT). Motivated by the potential of ZrO2 polymorphs to substitute for SiO2, we conducted calculations to ascertain their dielectric properties. A comprehensive analysis was conducted on all structural features, and their stability was assessed. ZrO2 polymorphs exhibit a wide bandgap with the type of bandgap also examined. Calculated zone-center phonon frequencies demonstrate the dynamical stability of ZrO2, with existing polymorphs showing strong agreement with experimental frequencies, particularly within the monoclinic polymorph. Raman and infrared (IR) spectra of ZrO2 polymorphs were simulated using density functional perturbation theory. ZrO2 demonstrates notable mechanical stability, as evidenced by calculated hardness (moduli), ductility, improved ductility, and higher elasticity. Calculated optical properties, including the dielectric constant and refractive index of ZrO2 polymorphs, play a pivotal role in optimizing their performance in various applications such as optoelectronic devices and antireflective materials.

• Krishnan, Manikandan; Ishigaki, Kento; Mariappan, Sathiskumar; Sokkalingam, Rajkumar; Gouchi, Jun & Bhoi, Dilip [Vis alle 12 forfattere av denne artikkelen] (2025). Evolution of superconducting and normal state properties of Fe1.09Se0.55Te0.45 under pressure. Journal of Physics and Chemistry of Solids. ISSN 0022-3697. 201. doi: 10.1016/j.jpcs.2025.112628. Fulltekst i vitenarkiv Vis sammendrag

  The Fe1+ySe1-xTex family of iron-based superconductors are extensively investigated for their unconventional nature of superconductivity, which arises from a complex interplay of spin and orbital ordering. At ambient conditions, Fe1.09Se0.55Te0.45 exhibits a superconducting transition below Tc∼14 K and a nematic ordering accompanied by a tetragonal to orthorhombic structural change at (Ts) which is marked by a sign change of Hall coefficient (RH) from positive to negative. In addition, the normal state resistivity follows a -log(T) increase with decreasing temperature due to the presence of excess Fe impurity acting as Kondo scattering centre. In this work, we investigate the evolution of superconducting and normal state properties of Fe1.09Se0.55Te0.45, a member of the Fe1+ySe1-xTex family, under hydrostatic pressure (P) using magneto-transport, dc magnetization and complementary first-principles band structure calculations. With applied P, the superconducting Tc reveals a dome-like shape, reaching a maximum Tc ∼19.9 K at critical pressure Pc ∼3.3 GPa. Simultaneously, with increasing pressure, both the -log(T) resistivity increase and Ts are gradually suppressed. Near Pc, Ts almost disappears, while the -log(T) resistivity increase persist beyond Pc up to 5 GPa and a Fermi liquid like behaviour emerges around 8 GPa. Furthermore, the band structure calculations suggest a pressure-induced structural change from orthorhombic to monoclinic symmetry near Pc. The nontrivial nature is evidenced by the effects of high pressure on the charge carrier balance, phase transition and superconductivity in Fe1.09Se0.55Te0.45. This nontrivial superconductivity is strongly linked to the significant normal state that arises from the connection between Fermi surface reconstruction and structural phase transitions.

• Wanniarachchige, Chapa Pamodani Wanniarachchi; Ponniah, Vajeeston; Rahman, Talal & Velauthapillai, Dhayalan (2025). First-Principles Insights into Mo and Chalcogen Dopant Positions in Anatase, TiO2. Computation. 13(7). doi: 10.3390/computation13070170. Fulltekst i vitenarkiv
• Tharuman, Sharmila; Nataraj, Nandini; Chen, Shen-Ming; Ponniah, Vajeeston & Vellaichamy, Balakumar (2025). Electrochemical sensing and catalytic water processing using CoFe-PBA: Experimental and DFT insights. Journal of Water Process Engineering. 71. doi: 10.1016/j.jwpe.2025.107301. Fulltekst i vitenarkiv Vis sammendrag

  The CoFe-PBA/GCE sensor was designed to detect 4-nitrophenol (4-NP) and exhibited excellent performance. Electrochemical impedance spectroscopy confirmed enhanced conductivity and improved charge transfer kinetics. Optimised redox behavior was observed through cyclic voltammetry, with calibration plots illustrating significant sensitivity and a detection limit of 0.015 μM obtained from differential pulse voltammetry. The sensor demonstrated selectivity by distinguishing 4-NP from interfering substances. Test for reproducibility and repeatability resulted in minimal relative standard deviations, emphasizing its dependability. DFT calculations indicated that the NO2 group of 4-NP primarily interacts with the Co site, with Bader analysis revealing charge flow direction. Overall, the CoFe-PBA/GCE sensor is a reliable and sensitive tool for detecting 4-NP. Additionally, 4-NP was catalytically reduced to 4-aminophenol (4-AP) using CoFe-PBA under 10 min using aqueous sodium borohydride (NaBH4). The prepared CoFe-PBA makes a highly effective catalyst for catalytic application and ecological remediation.

• Vignesh, A.; kumar, kumar Gnana; Ponniah, Vajeeston & Manthiram, Arumugam (2025). Bimetallic Organic Framework-Derived 3D Hierarchical Ni–Cu/MWCNTs as Anode Catalysts for High-Performance, Durable Direct Urea Fuel Cells. Advanced Energy Materials. ISSN 1614-6832. 15. doi: 10.1002/aenm.202405025. Fulltekst i vitenarkiv
• Mahalaxmi, Paraman; Balakrishnan, Kanimozhi; Veerapandy, Vasu; Nalini, Vajeeston & Ponniah, Vajeeston (2025). First-Principles Investigation of Pressure-Induced Structural Phase Transition and Properties of CsPbF3 Polymorphs. ACS Omega. 10(9), s. 9793–9807. doi: 10.1021/acsomega.5c01118. Fulltekst i vitenarkiv Vis sammendrag

  This study presents a first-principles investigation into the high-pressure studies of cesium lead fluoride (CsPbF3) polymorph using the Vienna ab initio simulation package (VASP). The CsPbF3 with Pm3̅m symmetry undergoes a pressure-induced structural transition, resulting in two distinct phases: R3̅c and Pnma. The structural stability, electronic structure, and optical properties of the three polymorphs of CsPbF3 (Pm3̅m, R3̅c and Pnma) are investigated using the plane wave pseudopotential method within the framework of density functional theory (DFT). The elastic constants and moduli of these polymorphs were computed and the result confirms that all are mechanically stable. Electronic band structure calculations indicate that all three CsPbF3 polymorphs exhibit semiconducting properties with a wide band gap (3–5 eV). The Pm3̅m, R3̅c form of CsPbF3 has a direct band gap while Pnma form has an indirect band gap. The mechanical stability and optical properties of the R3̅c and the Pnma phase of CsPbF3 have not been reported in the existing literature. By addressing this gap, this research contributes valuable data and sets the stage for future studies that explore these polymorphs in greater detail and their potential in advanced technological applications.

• Wanniarachchi, Chapa; Eidsvåg, Håkon; Arunasalam, Thevakaran; Ravirajan, Punniamoorthy; Velauthapillai, Dhayalan & Ponniah, Vajeeston (2024). Deep insight into structural and optoelectronic properties of mixed anion perovskites. Materials Research Express. 11(2). doi: 10.1088/2053-1591/ad1f98. Fulltekst i vitenarkiv Vis sammendrag

  Here we present the optoelectronic properties of pure inorganic lead-free halide perovskites in the form of Cs2AgBiX6 (X = Br, Cl, F, I) using the density functional theory calculations on cubic phase (Fm Misplaced & m) and tetragonal phase (I4/m). First, all the structures of the two phases were optimized at the PBE level. Structural, electronic, optical properties, phonon, and thermal properties of Cs2AgBiX6 in cubic (Fm Misplaced & m) and tetragonal phases (I4/m) were obtained using the VASP code. Tetragonal phases of all compounds of the form Cs2AgBiX6, except Cs2AgBiBr6, are reported here for the very first time. Among all the Cs2AgBiX6 (X = F, Cl, Br, I) structures, the cubic phase of Cs2AgBiBr6 was seen to have the highest absorption coefficient along with prominent electronic features that are favorable for optoelectronic applications. Thus, the cubic phase of Cs2AgBiBr6 was selected as the host lattice and bromine atoms were partly replaced with chlorine and iodine atoms. Electronic and optical properties of these mixed halide compounds of Cs2AgBiBr6−xFx, Cs2AgBiBr6−xClx, and Cs2AgBiBr6−xIx where x = 1, 2, 3, 4, 5 are investigated with hybrid functional HSE06 level. The electronic structure revealed that these mixed compounds exhibited indirect band gap nature regardless of the halide substitution (different x concentration) and the band gap of Cs2AgBiBr6 could be varied with the substitutions of fluorine, chlorine, and iodine atoms. Our in-depth analysis shows that Cs2AgBiBr6 and their mixed halides have the potential to become active double perovskite materials for photovoltaic applications and as photocatalysts for water splitting.

• Yartys, Volodymyr; Denys, Roman Volodymyrovich; Akselrud, Lev G.; Ponniah, Vajeeston; Dankelman, Robert & Plomp, Jeroen [Vis alle 11 forfattere av denne artikkelen] (2024). Structure and bonding in TiNiSi type LaMgSnH intermetallic hydride. Journal of Alloys and Compounds. ISSN 0925-8388. 976. doi: 10.1016/j.jallcom.2023.173198. Fulltekst i vitenarkiv
• Yartys, Volodymyr; Akselrud, Lev; Denys, Roman V.; Ponniah, Vajeeston; Ouladdiaf, Bachir & Dankelman, Robert [Vis alle 14 forfattere av denne artikkelen] (2024). Crystal, Magnetic Structures, and Bonding Interactions in the TiNiSi-Type Hydride CeMgSnH: Experimental and Computational Studies. Chemistry of Materials. ISSN 0897-4756. 36(12), s. 6257–6268. doi: 10.1021/acs.chemmater.4c01104. Fulltekst i vitenarkiv
• Ramesh, Karthikeyani; Venkidusamy, Sasirekha; Ponniah, Vajeeston; Venkatesan, Ragavendran & Mayandi, Jeyanthinath (2024). Biocompatible Triboelectric Nanogenerators for Self-Powered Microelectronics: Design, Performance, and Real-Time Applications. ACS Applied Bio Materials (AABM). doi: 10.1021/acsabm.4c01414. Fulltekst i vitenarkiv
• Devamanoharan, Arthi; Venkatachalapathy, Vishnukanthan; Veerapandy, Vasu & Ponniah, Vajeeston (2024). Investigating Stable Low-Energy Gallium Oxide (Ga2O3) Polytypes: Insights into Electronic and Optical Properties from First Principles. ACS Omega. 9(14), s. 16207–16220. doi: 10.1021/acsomega.3c10192. Fulltekst i vitenarkiv Vis sammendrag

  This study provides a comprehensive analysis of the electronic and optical properties of low-energy gallium oxide (Ga2O3) polytypes not considered earlier. Among these polytypes, the monoclinic structure (β-Ga2O3) holds significant relevance for both research and practical applications due to its superior stability under typical conditions. The primary aim of this research is to identify new and stable Ga2O3 polytypes that may exist under zero-temperature and zero-pressure conditions. To achieve this objective, we employ the VASP code to investigate electrical and optical properties, as well as stability assessments. Additionally, we examine phonon and thermal properties, including heat capacity, for all polytypes. This study also encompasses the computation of full elastic tensors and elastic moduli for all polytypes at 0 K, with Poisson’s and Pugh’s ratios confirming their ductile nature. Furthermore, we present the first ever report on the Raman- and infrared (IR)-active modes of these stable Ga2O3 polytypes. Our findings reveal that these mechanically and dynamically stable Ga2O3 polytypes exhibit semiconductive properties, as evidenced by electronic band structure investigations. This research offers valuable insights into the optical characteristics of Ga2O3 polytypes with potential applications spanning various fields.

• Eidsvåg, Håkon; Ponniah, Vajeeston & Velauthapillai, Dhayalan (2023). Doped MoS<inf>2</inf> Polymorph for an Improved Hydrogen Evolution Reaction. ACS Omega. 8(29), s. 26263–26275. doi: 10.1021/acsomega.3c02623. Fulltekst i vitenarkiv Vis sammendrag

  Green hydrogen produced from solar energy could be one of the solutions to the growing energy shortage as non-renewable energy sources are phased out. However, the current catalyst materials used for photocatalytic water splitting (PWS) cannot compete with other renewable technologies when it comes to efficiency and production cost. Transition-metal dichalcogenides, such as molybdenum disulfides (MoS2), have previously proven to have electronic and optical properties that could tackle these challenges. In this work, optical properties, the d-band center, and Gibbs free energy are calculated for seven MoS2 polymorphs using first-principles calculations and density functional theory (DFT) to show that they could be suitable as photocatalysts for PWS. Out of the seven, the two polymorphs 3Ha and 2R1 were shown to have d-band center values closest to the optimal value, while the Gibbs free energy for all seven polymorphs was within 5% of each other. In a previous study, we found that 3Hb had the highest electron mobility among all seven polymorphs and an optimal bandgap for photocatalytic reactions. The 3Hb polymorphs were therefore selected for further study. An in-depth analysis of the enhancement of the electronic properties and the Gibbs free energy through substitutional doping with Al, Co, N, and Ni was carried out. For the very first time, substitutional doping of MoS2 was attempted. We found that replacing one Mo atom with Al, Co, I, N, and Ni lowered the Gibbs free energy by a factor of 10, which would increase the hydrogen evolution reaction of the catalyst. Our study further shows that 3Hb with one S atom replaced with Al, Co, I, N, or Ni is dynamically and mechanically stable, while for 3Hb, replacing one Mo atom with Al and Ni makes the structure stable. Based on the low Gibbs free energy, stability, and electronic bandgap 3Hb, MoS2 doped with Al for one Mo atom emerges as a promising candidate for photocatalytic water splitting.

• Mariappan, Sathiskumar; Bhoi, Dilip; Krishnan, Manikandan; Shoko, Nagasaki; Ponniah, Vajeeston & Arushi, NN [Vis alle 10 forfattere av denne artikkelen] (2023). Electronic properties of α -Mn-type non-centrosymmetric superconductor Re<inf>5.5</inf>Ta under hydrostatic pressure. Superconductors Science and Technology. ISSN 0953-2048. 36(2). doi: 10.1088/1361-6668/aca83e. Fulltekst i vitenarkiv
• Pappathi, Murugan; Ponniah, Vajeeston; Raja, Antony Samy Sahaya; Vignesh, Asokan; Syed, Asad & Bahkali, Ali H. [Vis alle 11 forfattere av denne artikkelen] (2023). Trimetallic ZIF-Derived Ni-Cu/NC Rhombic Dodecahedron Nanostructures on Butter Sheet Paper as a Flexible Electrochemical Probe for Nonenzymatic Glucose Sensors. ACS Applied Nano Materials. 6(16), s. 14628–14644. doi: 10.1021/acsanm.3c01489. Fulltekst i vitenarkiv
• Balakrishnan, Kanimozhi; Veerapandy, Vasu & Ponniah, Vajeeston (2023). The optical properties and spectroscopic limited maximum efficiency of SnO<inf>2</inf> polymorphs for solar cell applications: A first-principles analysis. Computational condensed matter. ISSN 2352-2143. 37. doi: 10.1016/j.cocom.2023.e00856. Fulltekst i vitenarkiv
• Yartys, Volodymyr; Ponniah, Vajeeston; Denys, Roman Volodymyrovich; Havela, L.; Maskova-Cerna, S. & Szytula, A. (2023). Bonding mechanism and magnetic ordering in Laves phase λ1−MgCo2 intermetallic compound from theoretical and experimental studies. Scripta Materialia. ISSN 1359-6462. 237. doi: 10.1016/j.scriptamat.2023.115709. Fulltekst i vitenarkiv Vis sammendrag

  In the present work we have studied the bonding mechanism and magnetism in a hexagonal λ1−MgCo2 Laves phase intermetallic formed at rA/rB= 1.280. Ab initio theoretical calculations for MgCo2 using the projector-augmented wave method implemented in the VASP code showed that Mg carries a formal charge +1.7 |e| while Co atoms are also positively charged with around +0.7 |e| being donated to establish the interatomic covalent Co-Co bonding in a three-dimensional network of Co tetrahedra. MgCo2 is a metallic conductor with ferromagnetic properties and TC=303 K. Neutron Powder Diffraction study showed a collinear ordering of the Co moments, their alignment along the c-axis of the hexagonal unit cell with an average moment of 1.51(2) μB/atom Co. Ab initio computational studies indicate that the Co moments reach the experimentally observed value approx.1.5 μB when the unit cell volume exceeds 160 Å3 (average Co-Co distances 2.43 Å).

• 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.

• Ganesan, Kalaiselvan; Ponniah, Vajeeston; Yao, Jiyong; Vohra, Yogesh K. & Sonachalam, Arumugam (2023). Appearance of semiconductor-to-metal transition in Ba<inf>2</inf>BiFeSe<inf>5</inf> single crystal: A combined DFT and transport analysis under pressure. Journal of Materials Research. ISSN 0884-2914. doi: 10.1557/s43578-023-01235-y. Fulltekst i vitenarkiv
• Wanniarachchi, Chapa; Arunasalam, Thevakaran; Ravirajan, Punniyamoorthy; Velauthapillai, Dhayalan & Ponniah, Vajeeston (2023). Hybrid Functional Study on Electronic and Optical Properties of the Dopants in Anatase TiO<inf>2</inf>. ACS Omega. 8(45), s. 42275–42289. doi: 10.1021/acsomega.3c04329. Fulltekst i vitenarkiv Vis sammendrag

  TiO2 was known as a golden heterogeneous photocatalyst due to its chemical stability, low cost, nontoxicity, and strong oxidizing power. However, anatase TiO2 predominantly absorbs the photon energy in the ultraviolet region (λ < 387.5 nm); therefore, to increase the utilization of sunlight, the approach of doping of metals and nonmetals into pure TiO2 is implemented. Here we incorporate the dopants of Zr, Si, V, W, Ge, Cr, Sn, Mo, and Pb into the TiO2 lattice and study the optoelectronic properties, including the formation energies and the electron charge distributions, using the Vienna ab initio Simulation Package (VASP) from the hybrid functional of Heyd, Scuseria, and Erhzerhof (HSE06). We observed that V-, Mo-, and Cr-doped systems introduce shallow impurity states within the band gap, and those states influence the shift of the absorbance spectra to visible light by enhancing the photocatalytic efficiency. W-doped anatase TiO2 structure reduces the band gap of the pure anatase TiO2 by 0.7 eV. Notably, this reduction occurs without the introduction of any impurity states between the band edges. Additionally, the absorption edge of the solar spectrum shifts toward lower photon energy from 3.5 to 3.1 eV. From Bader charge analysis, we observed that mainly the charge transfer occurred from the dopants and charge accumulation happened around nearby oxygen atoms. The ferromagnetism was observed in V-, Cr-, Mo-, and W-doped anatase TiO2 structures due to the charge imbalance of the spin-up and spin-down states.

• Devamanoharan, Arthi; Veerapandy, Vasu & Ponniah, Vajeeston (2023). Structural, Electronic Properties, and Relative Stability Studies of Low-Energy Indium Oxide Polytypes Using First-Principles Calculations. ACS Omega. 8(14), s. 12928–12943. doi: 10.1021/acsomega.3c00105. Fulltekst i vitenarkiv Vis sammendrag

  Materials made of indium oxide (In2O3) are now being used as a potential component of the next generation of computers and communication devices. Density functional theory is used to analyze the physical, electrical, and thermodynamical features of 12 low-energy bulk In2O3 polytypes. The cubic structure In2O3 is majorly used for many of the In2O3-based transparent conducting oxides. The objective of this study is to explore other new stable In2O3 polytypes that may exist. The structural properties and stability studies are performed using the Vienna ab initio simulation package code. All the In2O3 polytypes have semiconductive properties, according to electronic band structure investigations. The full elastic tensors and elastic moduli of all polytypes at 0 K are computed. Poisson’s and Pugh’s ratio confirms that all stable polytypes are ductile. The phonon and thermal properties including heat capacity are obtained for mechanically stable polytypes. For the first time, we report the Raman and infrared active modes of stable polytypes.

• Devamanoharan, Arthi; Veerapandy, Vasu & Ponniah, Vajeeston (2022). Computational study on electronic and thermal stability of low energy indium oxide polytypes. Materials Today: Proceedings. 64, s. 1803–1807. doi: 10.1016/j.matpr.2022.06.078. Fulltekst i vitenarkiv Vis sammendrag

  The electronic properties of the indium oxide (In2O3) material make it suitable for solar cells, sensors and electrocatalysts. Structural, electronic and thermal properties of two bulk In2O3 polytypes with low energy are studied and investigated using density functional theory. From structural optimization based on total energy calculations, lattice and positional parameters have been established. The electronic properties were computed by HSE-06. The thermal properties including heat capacity and entropy were obtained. Evaluated the mechanical stability by computing the single-crystal elastic constants. Calculated the Poisson’s ratio and Bulk/Shear modulus which are above the critical value, implying that they are ductile material.

• Balakrishnan, Kanimozhi; Veerapandi, Vasu & Ponniah, Vajeeston (2022). High pressure structural phase transition and thermodynamic properties of SnO<inf>2</inf> polymorphs: First principles calculation. Materials Today: Proceedings. 64, s. 1798–1802. doi: 10.1016/j.matpr.2022.06.075. Fulltekst i vitenarkiv
• Yartys, Volodymyr; Berezovets, Vasyl; Ponniah, Vajeeston; Akselrud, Lev G.; Antonov, Vladimir & Fedotov, Vladimir [Vis alle 13 forfattere av denne artikkelen] (2022). Hydrogen induced structural phase transformation in ScNiSn-based intermetallic hydride characterized by experimental and computational studies. Acta Materialia. ISSN 1359-6454. 244. doi: 10.1016/j.actamat.2022.118549. Fulltekst i vitenarkiv Vis sammendrag

  Understanding an interrelation between the structure, chemical composition and hydrogenation properties of intermetallic hydrides is crucial for the improvement of their hydrogen storage performance. Ability to form the hydrides and to tune the thermodynamics and kinetics of their interaction with hydrogen is related to their chemical composition. Some features of the metal–hydrogen interactions remain however poorly studied, including chemistry of Sc-containing hydrides. ZrNiAl-type ScNiSn-based intermetallic hydride has been probed in the present work using a broad range of experimental techniques including Synchrotron and Neutron Powder Diffraction, 119Sn Möessbauer Spectroscopy, hydrogenation at pressures reaching several kbar H2 and hydrogen Thermal Desorption Spectroscopy studies. Computational DFT calculations have been furthermore performed. This allowed to establish the mechanism of the phase-structural transformation and electronic structure changes causing a unique contraction of the metal lattice of intermetallic alloy and the formation of the ...H-Ni-H-Ni… chains in the structure with H atoms carrying a partial negative charge. Such hydrogen absorption accompanied by a formation of a covalent Ni-H bonding and causing an unusual behavior contracts to the conventionally observed bonding mechanism of hydrogen in metals as based on the metallic bonding frequently accompanied by a jumping diffusion movement of the inserted H atoms – in contrast to the directional Metal-Hydrogen bonding observed in the present work. At high applied pressures ScNiSnH0.83 orthorhombic TiNiSi type hydride is formed with H atoms filling Sc3Ni tetrahedra. Finally, this study shows that scandium closely resembles the behavior of the heavy rare earth metal holmium.

• Mayandi, Jeyanthinath; Schrade, Matthias; Ponniah, Vajeeston; Stange, Marit Synnøve Sæverud; Lind, Anna Maria & Sunding, Martin Fleissner [Vis alle 13 forfattere av denne artikkelen] (2022). High entropy alloy CrFeNiCoCu sputter deposited films: Structure, electrical properties, and oxidation. Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. ISSN 0734-2101. 40(2). doi: 10.1116/6.0001394. Fulltekst i vitenarkiv Vis sammendrag

  High entropy alloy (HEA) films of CrFeCoNiCu were deposited by sputtering. Their structure was characterized and their electric transport properties were studied by temperature-dependent Hall and Seebeck measurements. The HEA films show a solid solution with an fcc structure. The residual electrical resistivity of the films is around 130 μΩ cm, which is higher than the Mott limit for a metal while the temperature dependence of the resistivity above 30 K is metal-like but with a small temperature coefficient of resistivity (2 ppm/K). The dominant scattering mechanism of charge carriers is alloy scattering due to chemical disorder in the HEA. The Hall coefficient is positive while the Seebeck coefficient is negative. This is interpreted as arising from an electronic structure where the Fermi level passes through band states having both holes and electrons as indicated by band structure calculations. Below 30 K, the conduction is discussed in terms of weak localization and Kondo effects. The HEA structure appears stable for annealing in vacuum, while annealing in an oxygen-containing atmosphere causes the surface to oxidize and grow a Cr-rich oxide on the surface. This is then accompanied by demixing of the HEA solid solution and a decrease in the effective resistance of the film.

• 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.

• Lingannan, Govindaraj; Joseph, Boby; Kuo, Chia Nung; Lue, Chin Shan; Ponniah, Vajeeston & Sonachalam, Arumugam (2022). Differing pressure response of lattice structure in LaTMSb<inf>2</inf> (TM = Au or Ag) ternary antimonides. Bulletin of Materials Science. ISSN 0250-4707. 45(4). doi: 10.1007/s12034-022-02778-2. Fulltekst i vitenarkiv
• Mariappan, Sathiskumar; Krishnan, Manikandan; Bhoi, Dilip; Ma, Hanming; Gouchi, Jun & Motla, Kapil [Vis alle 10 forfattere av denne artikkelen] (2022). Superconducting and structural properties of the noncentrosymmetric Re6Hf superconductor under high pressure. Physical review B (PRB). ISSN 2469-9950. 105(22). doi: 10.1103/PhysRevB.105.224505. Fulltekst i vitenarkiv
• 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.

• Mayandi, Jeyanthinath; Finstad, Terje Gunnar; Dahl, Øystein; Ponniah, Vajeeston; Schrade, Matthias & Løvvik, Ole Martin [Vis alle 8 forfattere av denne artikkelen] (2022). Thin films made by reactive sputtering of high entropy alloy FeCoNiCuGe: Optical, electrical and structural properties. Thin Solid Films. ISSN 0040-6090. 744(139083). doi: 10.1016/j.tsf.2022.139083. Fulltekst i vitenarkiv Vis sammendrag

  Films were reactively sputtered from a high entropy alloy (HEA) FeCoNiCuGe target in an Ar/O2 plasma. The case of zero O2 gas flow yielded HEA films of FeCoNiCuGe. These as-deposited HEA films have a face centered cubic (FCC) structure, showing a texture with columnar grains containing many planar defects. The residual electrical resistivity of the films is around 225 μΩcm and the temperature dependence of the resistivity is metal-like. The temperature coefficient of resistivity is small (4.5 ppm/K). The Hall coefficient is positive while the Seebeck coefficient is negative. This is interpreted as arising from an electronic structure having both holes and electrons at the Fermi level as indicated by band structure calculations. The HEA FCC structure is unstable upon annealing in forming gas and showed demixing. Annealing in O2 also yielded inhomogeneous oxides, with a thick layer of CuO growing on the surface. The cases of reactive sputtering with an oxygen flow yielded oxides that are either nanocrystalline or amorphous dependent upon the sputter conditions. These can be classified as high entropy oxides (HEO) and have an optical bandgap around 1.9 eV and high transmission in the infrared region. The amorphous HEO has an electrical conduction interpreted as due to variable range hopping described by the Efros–Shklovskii theory. The HEO films were reduced in forming gas. For the amorphous HEO film, the reduction at 300–500 °C yielded hexagonal Ni5Ge2 and an FCC phase. For the nanocrystalline HEO, reduction resulted in creation of FCC and body centered cubic HEA metal phases.

• Banu, Banu Lathifa; Veerapandy, Vasu & Ponniah, Vajeeston (2022). Structural stability and electronical properties of MoSe<inf>2</inf> polymorphs: DFT. Materials Today: Proceedings. 64, s. 1808–1812. doi: 10.1016/j.matpr.2022.06.079. Fulltekst i vitenarkiv
• 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
• Azarov, Alexander; Bazioti, Kalliopi; Venkatachalapathy, Vishnukanthan; Ponniah, Vajeeston; Monakhov, Eduard & Kuznetsov, Andrej (2022). Disorder-Induced Ordering in Gallium Oxide Polymorphs. Physical Review Letters. ISSN 0031-9007. 128(1). doi: 10.1103/PhysRevLett.128.015704. Fulltekst i vitenarkiv Vis sammendrag

  olymorphs are common in nature and can be stabilized by applying external pressure in materials. The pressure and strain can also be induced by the gradually accumulated radiation disorder. However, in semiconductors, the radiation disorder accumulation typically results in the amorphization instead of engaging polymorphism. By studying these phenomena in gallium oxide we found that the amorphization may be prominently suppressed by the monoclinic to orthorhombic phase transition. Utilizing this discovery, a highly oriented single-phase orthorhombic film on the top of the monoclinic gallium oxide substrate was fabricated. Exploring this system, a novel mode of the lateral polymorphic regrowth, not previously observed in solids, was detected. In combination, these data envisage a new direction of research on polymorphs in Ga2O3 and, potentially, for similar polymorphic families in other materials.

• Wanniarachchi, Chapa; Eidsvåg, Håkon; Arunasalam, Thevakaran; Ravirajan, Punniamoorthy; Velauthapillai, Dhayalan & Ponniah, Vajeeston (2022). Cs2AgBiBr6 as a mixed anion perovskites for photovoltaic applications: A first-principle study. Materials Today: Proceedings. 64(Part 5), s. 1783–1788. doi: 10.1016/j.matpr.2022.06.063. 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.

• Selvapriya, R.; Sasirekha, V.; Vajeeston, Ponniah; Pearce, Joshua M. & Mayandi, J. (2021). Reaction induced multifunctional TiO2 rod/particle nanostructured materials for screen printed dye sensitized solar cells. Ceramics International. ISSN 0272-8842. 47(6), s. 8094–8104. doi: 10.1016/j.ceramint.2020.11.164. Fulltekst i vitenarkiv
• 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.

• Vignesh, A.; Ponniah, Vajeeston; Pannipara, Mehboobali; Al-Sehemi, Abdullah G.; Xia, Yang & kumar, G. Gnana (2021). Bimetallic metal-organic framework derived 3D hierarchical NiO/Co3O4/C hollow microspheres on biodegradable garbage bag for sensitive, selective, and flexible enzyme-free electrochemical glucose detection. Chemical Engineering Journal. ISSN 1385-8947. 430(4). doi: 10.1016/j.cej.2021.133157. Fulltekst i vitenarkiv
• Murugesan, Kannan; Lingannan, Govindaraj; Ishigaki, Kento; Uwatoko, Yoshiya; Sekine, Chihiro & Kawamura, Yukihiro [Vis alle 12 forfattere av denne artikkelen] (2021). Pressure Dependence of Superconducting Properties, Pinning Mechanism, and Crystal Structure of the Fe0.99Mn0.01Se0.5Te0.5 Superconductor. ACS Omega. 6(45), s. 30419–30431. doi: 10.1021/acsomega.1c03721. Fulltekst i vitenarkiv Vis sammendrag

  We have investigated the pressure (P) effect on structural (up to 10 GPa), transport [R(T): up to 10 GPa], and magnetic [(M(T): up to 1 GPa)] properties and analyzed the flux pinning mechanism of the Fe0.99Mn0.01Se0.5Te0.5 superconductor. The maximum superconducting transition temperature (Tc) of 22 K with the P coefficient of Tc dTc/dP = +2.6 K/GPa up to 3 GPa (dTc/dP = −3.6 K/GPa, 3 ≤ P ≥ 9 GPa) was evidenced from R(T) measurements. The high-pressure diffraction and density functional theory (DFT) calculations reveal structural phase transformation from tetragonal to hexagonal at 5.9 GPa, and a remarkable change in the unit cell volume is observed at ∼3 GPa where the Tc starts to decrease, which may be due to the reduction of charge carriers, as evidenced by a reduction in the density of states (DOS) close to the Fermi level. At higher pressures of 7.7 GPa ≤ P ≥ 10.2 GPa, a mixed phase (tetragonal + hexagonal phase) is observed, and the Tc completely vanishes at 9 GPa. A significant enhancement in the critical current density (JC) is observed due to the increase of pinning centers induced by external pressure. The field dependence of the critical current density under pressure shows a crossover from the δl pinning mechanism (at 0 GPa) to the δTc pinning mechanism (at 1.2 GPa). The field dependence of the pinning force at ambient condition and under pressure reveals the dense point pinning mechanism of Fe0.99Mn0.01Se0.5Te0.5. Moreover, both upper critical field (HC2) and JC are enhanced significantly by the application of an external P and change over to a high P phase (hexagonal ∼5.9 GPa) faster than a Fe0.99Ni0.01Se0.5Te0.5 (7.7 GPa) superconductor.

• Eidsvåg, Håkon; Rasukkannu, Murugesan; Velauthapillai, Dhayalan & Vajeeston, Ponniah (2021). In-depth first-principle study on novel MoS2 polymorphs. RSC Advances. 11(6), s. 3759–3769. doi: 10.1039/d0ra10443d. Fulltekst i vitenarkiv Vis sammendrag

  Molybdenum disulphide (MoS2) is a rising star among transition-metal dichalcogenides in photovoltaics, diodes, electronic circuits, transistors and as a photocatalyst for hydrogen evolution. A wide range of MoS2 polymorphs with varying electrical, optical and catalytic properties is of interest. However, in-depth studies on the structural stability of the various MoS2 polymorphs are still lacking. For the very first time, 14 different MoS2 polymorphs are proposed in this study and in-depth analysis of these polymorphs are carried out by employing first-principle calculations based on density functional theory (DFT). In order to investigate the feasibility of these polymorphs for practical applications, we employ wide range of analytical methods including band structure, phonon and elastic constant calculations. Three of the polymorphs were shown to be unstable based on the energy volume calculations. Among the remaining eleven polymorphs (1T1, 1T2, 1H, 2T, 2H, 2R1, 2R2, 3Ha, 3Hb, 3R and 4T), we confirm that the 1T1, 1T2, 2R2 and 3R polymorphs are not dynamically stable based on phonon calculations. Recent research suggests that stabilising dopants (e.g. Li) are needed if 1T polymorphs to be synthesised. Our study further shows that the remaining seven polymorphs are both dynamically and mechanically stable, which make them interesting candidates for optoelectronics applications. Due to high electron mobility and a bandgap of 1.95 eV, one of the MoS2 polymorphs (3Hb-MoS2) is proposed to be the most promising candidate for these applications.

• Eidsvåg, Håkon; Bentouba, Said; Vajeeston, Ponniah; Shivatharsiny, Yohi & Velauthapillai, Dhayalan (2021). TiO2 as a Photocatalyst for Water Splitting—An Experimental and Theoretical Review. Molecules. ISSN 1431-5157. 26(6). doi: 10.3390/molecules26061687. Fulltekst i vitenarkiv Vis sammendrag

  Abstract: Hydrogen produced from water using photocatalysts driven by sunlight is a sustainable way to overcome the intermittency issues of solar power and provide a green alternative to fossil fuels. TiO2 has been used as a photocatalyst since the 1970s due to its low cost, earth abundance, and stability. There has been a wide range of research activities in order to enhance the use of TiO2 as a photocatalyst using dopants, modifying the surface, or depositing noble metals. However, the issues such as wide bandgap, high electron-hole recombination time, and a large overpotential for the hydrogen evolution reaction (HER) persist as a challenge. Here, we review state-of-the-art experimental and theoretical research on TiO2 based photocatalysts and identify challenges that have to be focused on to drive the field further. We conclude with a discussion of four challenges for TiO2 photocatalysts—non-standardized presentation of results, bandgap in the ultraviolet (UV) region, lack of collaboration between experimental and theoretical work, and lack of large/small scale production facilities. We also highlight the importance of combining computational modeling with experimental work to make further advances in this exciting field.

• Govindaraj, Lingannan; Boby, Joseph; Vajeeston, Ponniah; Nung, Kuo Chia; Shan, Lue Chin & Ganesan, Kalaiselvan [Vis alle 8 forfattere av denne artikkelen] (2021). Pressure-dependent modifications in the LaAuSb2 charge density wave system. Physical review B (PRB). ISSN 2469-9950. 103. doi: 10.1103/PhysRevB.103.195126. Fulltekst i vitenarkiv Vis sammendrag

  Hydrostatic pressure response of LaAuSb2 charge density wave (CDW) system is investigated using electrical transport, x-ray diffraction (XRD), and density-functional theory (DFT). Resistivity data at ambient pressure evidence a clear CDW transition at 83 K. X-ray diffraction at ambient pressure reveals that in plane and out of plane axes show opposite behavior with decreasing temperature, in particular, out of plane c axis develops a distinct change at ∼250 K, much above the observed CDW transition at 83 K. The CDW transition shifts to low temperature with increasing pressure. Our resistivity data indicate a complete suppression of the CDW transition at ∼3.6 GPa. High-pressure XRD revealed a change from the linear trend for the out of plane (c) and the in plane (a) lattice parameters for pressure above 3.8 GPa. With compression, DFT indicated an anomaly in the c/a ratio around 8 GPa. The calculated electronic structure also indicated minor changes in the band structure in this pressure range. In addition, high-pressure DFT structural investigations reveal the LaAuSb2 system to be stable up to pressures as high as 150 GPa

• 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.

• Kalaiselvan, Ganesan; Govindaraj, Lingannan; Kannan, Murugesan; S., Perreault Christopher; K., Samudrala Gopi & Kumar, Maheshwari Pankaj [Vis alle 10 forfattere av denne artikkelen] (2021). Pressure‑induced structural transition and huge enhancement of superconducting properties of single‑crystal Fe0.99Ni0.01Se0.5Te0.5 unconventional superconductor. Journal of Materials Research. ISSN 0884-2914. 36, s. 1624–1636. doi: 10.1557/s43578-021-00110-y. Fulltekst i vitenarkiv Vis sammendrag

  We report high-pressure structural studies (52 GPa) at room temperature combined with magnetic [(M(T):1GPa] and electrical resistivity [(ρ(T):0-21GPa)] measurements down to 2 K on Fe0.99Ni0.01Se0.5Te0.5 superconductor using designer diamond anvils (D-DAC) pressure cell. The M(T) data show huge enhancement of superconducting transition temperature (Tc) from 8.62 to 14.8 K (1 GPa) and ρ(T) reveal maximum enhancement of Tc ~ 30.5 K at 3 GPa (dTc/dP =  ~ 7.19 K/GPa) followed by moderate decrease of Tc up to 19 K at 7.5 GPa, and further increasing pressure Tc gets vanished at 10.6 GPa. The reduction of Tc due to the occurrence of structural transition that is likely associated with possible reduction of charge carriers in the density of states in Fermi surface. The high-pressure XRD measurement shows tetragonal phase exists up to 7 GPa, followed by mixed phase which is visible between 7.5 GPa and 14.5 GPa. The structural transformation occurs at 15 GPa from tetragonal (P4/nmm) to NiAs -type hexagonal phase (P63/mmc) and it is stable up to 52 GPa, confirmed from the equation of state (EOS) and it can be correlated with variation of Tc under pressure for Fe0.99Ni0.01Se0.5Te0.5 chalcogenide superconductors.

• 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.

• Mayandi, Jeyanthinath; Finstad, Terje; Venkatesan, Ragavendran; Vajeeston, Ponniah; Karazhanov, Smagul & Venkatachalapathy, Vishnukanthan (2020). Carbon-dioxide as annealing atmosphere to retain the electrical properties of indium-tin oxide. Materials Letters. ISSN 0167-577X. 276. doi: 10.1016/j.matlet.2020.128195. Fulltekst i vitenarkiv Vis sammendrag

  In practical applications of indium-tin-oxide (ITO) annealing at temperatures ~400 °C without degrading its electrical and optical properties is an important challenge. In the present work, commercial Indium-tin oxide (ITO) coated on glass was subjected to post-annealing treatment in the range of 200-400 °C at different annealing atmospheres; oxygen, nitrogen and carbon-dioxide. The annealed samples were characterized by X-ray diffraction, UV-visible spectroscopy and Hall measurements to evaluate the structural, optical and electrical properties. Both oxygen and nitrogen treated samples degrades the structural, optical and conducting properties of ITO, while carbon-dioxide atmosphere inhibits the degradation of ITO at 400 °C. The obtained results suggest that carbon-dioxide can be well utilized as annealing ambient to retain opto-electronic, structural and electrical properties of ITO and thereby improve the efficiency of ITO based solar cells.

• 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.

• 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.

• Rasukkannu, Murugesan; Velauthapillai, Dhayalan & Vajeeston, Ponniah (2019). First-Principle Calculation of High Absorption-TlGaTe2 for Photovoltaic Application. Materials. 12(7). doi: 10.3390/ma12172667. Fulltekst i vitenarkiv Vis sammendrag

  We use first-principle calculations based on hybrid functional and the Bethe-Salpeter equation method to investigate the electronic and optical properties of dichalcogenide TlGaTe2. Based on theoretical studies, TlGaTe2 has until recently been considered as an indirect band gap material, however; by employing more accurate hybrid functional model, we showed that although TlGaTe2 has an indirect band gap of 1.109 eV, it also exhibits a fundamental direct band gap of 1.129 eV. Our finding was further confirmed by the optical studies on TlGaTe2, which show that the absorption peak is registered at a photon energy of 1.129 eV. It was also shown that TlGaTe2 has high optical absorption peaks in the visible region. Based on phonon and elastic constant calculations, it was shown that TlGaTe2 is dynamically and mechanically stable. Our findings show that TlGaTe2 is a potential candidate for photovoltaic application.

• Vajeeston, Ponniah; Bianchini, Federico & Fjellvåg, Helmer (2019). First-Principles Study of the Structural Stability and Dynamic Properties of Li2MSiO4 (M = Mn, Co, Ni) Polymorphs. Energies. 12(224), s. 1–24. doi: 10.3390/en12020224. Fulltekst i vitenarkiv Vis sammendrag

  In recent years, the scientific community has shown an increasing interest in regards to the investigation of novel materials for the intercalation of lithium atoms, suitable for application as cathodes in the new generations of Li-ion batteries. Within this framework, we have computed the relative structural stability, the electronic structure, the elastic and dynamic properties of Li2MSiO4 compounds (M = Mn, Co, Ni) by means of first-principles calculations based on density functional theory. The so-obtained structural parameters of the examined phases are in agreement with previous reports. The energy differences between different polymorphs are found to be small, and most of these structures are dynamically stable. The band structures and density of states are computed to analyse the electronic properties and characterise the chemical bonding. The single crystal elastic constants are calculated for all the examined modifications, proving their mechanical stability. These Li2MSiO4 materials are found to present a ductile behaviour upon deformation. The diffusion coefficients of Li ions, calculated at room temperature for all the examined modifications, reveal a poor conductivity for this class of materials.

• 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.

• Li, Xinyu; Bianchini, Federico; Wind, Julia; Vajeeston, Ponniah; Wragg, David & Fjellvåg, Helmer (2019). P2 Type Layered Solid-State Electrolyte Na2Zn2TeO6: Crystal Structure and Stacking Faults. Journal of the Electrochemical Society. ISSN 0013-4651. 166(15), s. A3830–A3837. doi: 10.1149/2.1231915jes. Fulltekst i vitenarkiv
• Ruud, Amund; Sottmann, Jonas; Vajeeston, Ponniah & Fjellvåg, Helmer (2019). Direct observation of reversible conversion and alloying reactions in a Bi2(MoO4)3-based lithium-ion battery anode. Journal of Materials Chemistry A. ISSN 2050-7488. 7(30), s. 17906–17913. doi: 10.1039/c9ta03459e. Fulltekst i vitenarkiv
• Yacov, Finkelstein; Raymond, Moreh; Bianchini, Federico & Vajeeston, Ponniah (2019). Anisotropy of the proton kinetic energy in ice Ih. Surface Science. ISSN 0039-6028. 679, s. 174–179. doi: 10.1016/j.susc.2018.09.010. Fulltekst i vitenarkiv Vis sammendrag

  The partial vibrational density of states (pVDOS) of ice Ih, as simulated by first principle modeling based on density functional theory (DFT), is utilized for computing the Cartesian components of the proton and oxygen quantum kinetic energies, Ke(H) and Ke(O) respectively, along and perpendicular to the hydrogen bonds. The DFT method was found to yield better agreement with deep inelastic neutron scattering (DINS) measurements than the semi empirical (SE) calculations. The advantage of using the DFT method is to enable us to resolve the external and internal phonon bands of the Cartesian projections of the pVDOS, and hence those of the lattice and vibrational components of Ke(H). We show that a pVDOS analysis is a valuable tool in testing scattering results of complex systems and suggest its potential to explore competing quantum effects, e.g. on Ke(H) across phase transitions in water.

• Bianchini, Federico; Fjellvåg, Helmer & Vajeeston, Ponniah (2019). Nonhexagonal Na Sublattice Reconstruction in the Super-Ionic Conductor Na2Zn2TeO6: Insights from Ab Initio Molecular Dynamics. Journal of Physical Chemistry C. ISSN 1932-7447. 123(8), s. 4654–4663. doi: 10.1021/acs.jpcc.8b10362. Fulltekst i vitenarkiv
• Vajeeston, Ponniah (2018). Ionic conductivity enhancement by particle size reduction in Li2FeSiO4. Materials Letters. ISSN 0167-577X. 218, s. 313–316. doi: 10.1016/j.matlet.2018.02.041. Fulltekst i vitenarkiv
• El-Kharbachi, Abdelouahab; Hu, Yang; Yoshida, Koji; Vajeeston, Ponniah; Kim, Sangryun & Sørby, Magnus Helgerud [Vis alle 9 forfattere av denne artikkelen] (2018). Lithium ionic conduction in composites of Li(BH4)0.75I0.25 and amorphous 0.75Li2S·0.25P2S5 for battery applications. Electrochimica Acta. ISSN 0013-4686. 278, s. 332–339. doi: 10.1016/j.electacta.2018.05.041. Fulltekst i vitenarkiv
• Vajeeston, Ponniah; Fjellvåg, Helmer & Nilsen, Ola (2018). Search for potential precursors for Si-atomic layer deposition- a quantum chemical study. Materials Letters. ISSN 0167-577X. 216, s. 189–192. doi: 10.1016/j.matlet.2018.01.040. Fulltekst i vitenarkiv Vis sammendrag

  Thin film of silicon is an interesting material for many technological applications in electronic industry and in energy harvesting technologies, but requires a method for controlled growth of thin films. The purpose of this study is to screen a wide variety of Si content precursors for Si atomic layer deposition (ALD) reactions using state-of-the-art density-functional calculations. Among the studied 85 Si content precursors we found that C7H12OSi–Methoxy-trivinyl-silane and C7H9NSi–Benzyliminosilane show positive indications for ALD reactivity for Si deposition. We believe that this finding will be helpful to develop low-cost, high-energy efficiency thin-film solar cells for future scale up implementation in photovoltaics.

• Eidsvåg, Håkon; Rasukkannu, Murugesan; Vajeeston, Ponniah & Velauthapillai, Dhayalan (2018). Bandgap engineering in CsSnxPb(1−x)I3 and their influence on light absorption. Materials Letters. ISSN 0167-577X. 218, s. 253–256. doi: 10.1016/j.matlet.2018.02.021. Fulltekst i vitenarkiv
• Rasukkannu, Murugesan; Velauthapillai, Dhayalan & Vajeeston, Ponniah (2018). A first-principle study of the electronic, mechanical and optical properties of inorganic perovskite Cs2SnI6 for intermediate-band solar cells. Materials Letters. ISSN 0167-577X. 218, s. 233–236. doi: 10.1016/j.matlet.2018.02.034. Fulltekst i vitenarkiv
• Rasukkannu, Murugesan; Velauthapillai, Dhayalan; Bianchini, Federico & Vajeeston, Ponniah (2018). Properties of novel non-silicon materials for photovoltaic applications: A first-principle insight. Materials. 11:2006(10), s. 1–17. doi: 10.3390/ma11102006. Fulltekst i vitenarkiv Vis sammendrag

  Due to the low absorption coefficients of crystalline silicon-based solar cells, researchers have focused on non-silicon semiconductors with direct band gaps for the development of novel photovoltaic devices. In this study, we use density functional theory to model the electronic structure of a large database of candidates to identify materials with ideal properties for photovoltaic applications. The first screening is operated at the GGA level to select only materials with a sufficiently small direct band gap. We extracted twenty-seven candidates from an initial population of thousands, exhibiting GGA band gap in the range 0.5–1 eV. More accurate calculations using a hybrid functional were performed on this subset. Based on this, we present a detailed first-principle investigation of the four optimal compounds, namely, TlBiS2, Ba3BiN, Ag2BaS2, and ZrSO. The direct band gap of these materials is between 1.1 and 2.26 eV. In the visible region, the absorption peaks that appear in the optical spectra for these compounds indicate high absorption intensity. Furthermore, we have investigated the structural and mechanical stability of these compounds and calculated electron effective masses. Based on in-depth analysis, we have identified TlBiS2, Ba3BiN, Ag2BaS2, and ZrSO as very promising candidates for photovoltaic applications.

• Bianchini, Federico; Fjellvåg, Helmer & Vajeeston, Ponniah (2018). A first-principle study of NaMPO4(M = Mn, Fe, Co, Ni) possible novelstructures as cathode materials for sodium-ion batteries: Structural andelectrochemical characterisation. Materials Chemistry and Physics. ISSN 0254-0584. 219, s. 212–221. doi: 10.1016/j.matchemphys.2018.08.007. Fulltekst i vitenarkiv Vis sammendrag

  Transition metal containing polyanion compounds are effective excellent electrode materials for sodium-ion batteries due to their high intrinsic electrochemical potentials and to the resulting high energy density. Iron sodium phosphates, in particular, are attractive due to the large natural abundance of both Na and Fe. These materials have been extensively studied in their most common olivine structures: maricite and triphylite. In this work, we expand the current knowledge of this class of materials by investigating the structural properties and the energetics of a series of modification exhibiting different coordination for the intermetallic atom M = Mn, Fe, Co, Ni by means of density functional theory calculations. An expanded-volume NaFePO4 configuration with the zeolite ABW structure is predicted to be stable at high temperature. This type of structure, presenting a tetrahedral FeO coordination geometry, has been previously reported only for the NaCoPO4 case. A semi-amorphous phase is predicted to be a possible metastable intermediate configuration between the known octahedral coordinated structures and the novel tetrahedral-coordinated one. The electrochemical characterisation of the latter reveals a similar deintercalation potential with respect to triphylite, and a higher diffusion barrier caused by the incompressibility of the PO4 tetrahedra along the diffusive path. This result offers important insight about the correlation between the diffusive properties of ions and their local chemical environment.

• Rasukkannu, Murugesan; Velauthapillai, Dhayalan & Vajeeston, Ponniah (2018). A promising high-efficiency photovoltaic alternative non-silicon material: A first-principle investigation. Scripta Materialia. ISSN 1359-6462. 156, s. 134–137. doi: 10.1016/j.scriptamat.2018.07.027. Fulltekst i vitenarkiv
• Rasukkannu, Murugesan; Velauthapillai, Dhayalan & Vajeeston, Ponniah (2018). Hybrid Density Functional Study of Au2Cs2I6, Ag2GeBaS4, Ag2ZnSnS4, and AgCuPO4 for the Intermediate Band Solar Cells. Energies. 11(12). doi: 10.3390/en11123457. Fulltekst i vitenarkiv Vis sammendrag

  We present a comprehensive investigation of the structural, electronic, mechanical, and optical properties of four promising candidates, namely Au2Cs2I6, Ag2GeBaS4, Ag2ZnSnS4, and AgCuPO4, for application in photovoltaic devices based on intermediate band (IB) cells. We perform accurate density functional theory calculations by employing the hybrid functional of Heyd, Scuseria, and Erhzerhof (HSE06). Calculations reveal that IBs are present in all proposed compounds at unoccupied states in the range of 0.34–2.19 eV from the Fermi level. The structural and mechanical stability of these four materials are also systematically investigated. Additional peaks are present in the optical spectra of these compounds, as characterised by a broadened energy range and high intensity for light absorption. Our findings, as reported in this work, may provide a substantial breakthrough on the understanding of these materials, and thus help the design of more efficient IB solar devices.

• Bianchini, Federico; Fjellvåg, Helmer & Vajeeston, Ponniah (2018). A first principle comparative study of the ionic diffusivity in LiAlO2 and NaAlO2 polymorphs for solid-state battery applications. Physical Chemistry, Chemical Physics - PCCP. ISSN 1463-9076. 20(15), s. 9824–9832. doi: 10.1039/c8cp00715b. Fulltekst i vitenarkiv
• 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.

• Bianchini, Federico; Fjellvåg, Helmer & Vajeeston, Ponniah (2018). A first-principles investigation of the Li difusion mechanism in the super-ionic conductor lithium orthothioborate Li3BS3 structure. Materials Letters. ISSN 0167-577X. 219, s. 186–189. doi: 10.1016/j.matlet.2018.02.083. Fulltekst i vitenarkiv
• Ruud, Amund; Sottmann, Jonas; Vajeeston, Ponniah & Fjellvåg, Helmer (2018). Operando Investigations of Lithiation and Delithiation Processes in the BiVO4 Anode Material. Physical Chemistry, Chemical Physics - PCCP. ISSN 1463-9076. 47, s. 29798–29803. doi: 10.1039/c8cp05330h. Fulltekst i vitenarkiv

Se alle arbeider i NVA

• 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
• Håkon, Eidsvåg; Murugesan, Rasukkannu; Dhayalan, Velauthapillai & Ponniah, Vajeeston (2021). In-depth first-principle study on novel MoS2 polymorphs. Fulltekst i vitenarkiv Vis sammendrag

  Introduction: Molybdenum disulphide (MoS2) is a rising star among transition-metal dichalcogenides in photovoltaics, diodes, electronic circuits, transistors and as a photocatalyst for hydrogen evolution. A wide range of MoS2 polymorphs with varying electrical, optical and catalytic properties is of interest. However, in-depth studies on the structural stability of the various MoS2 polymorphs are still lacking. Methods and Results: For the very first time, 14 different MoS2 polymorphs are proposed in this study and in-depth analysis of these polymorphs are carried out by employing first-principle calculations based on density functional theory (DFT). In order to investigate the feasibility of these polymorphs for practical applications, we employ wide range of analytical methods including band structure, phonon and elastic constant calculations. Three of the polymorphs were shown to be unstable based on the energy volume calculations. Among the remaining eleven polymorphs (1T1, 1T2, 1H, 2T, 2H, 2R1, 2R2, 3Ha, 3Hb, 3R and 4T), we confirm that the 1T1, 1T2, 2R2 and 3R polymorphs are not dynamically stable based on phonon calculations. Recent research suggests that stabilising dopants (e.g. Li) are needed if 1T polymorphs to be synthesised. Conclusions: Our study further shows that the remaining seven polymorphs are both dynamically and mechanically stable, which make them interesting candidates for optoelectronics applications. Due to high electron mobility and a bandgap of 1.95 eV, one of the MoS2 polymorphs (3Hb-MoS2) is proposed to be the most promising candidate for these applications.

• Kanimozhi, Balakrishnan; Velauthapillai, Vasu & Ponniah, Vajeeston (2021). Physical and chemical properties of some newly identified polymorphs of SnO2: Density functional theory. Fulltekst i vitenarkiv Vis sammendrag

  For the first time, we investigated the structural stability, and ground state properties of eleven polymorphs in the sequence of experimental phases determined by density functional theory (DFT). Among the eleven phases Pnmb (62) structure is dynamically and mechanically unstable and Fm m and I4_1/amd are dynamically unstable, where the remaining eight phases satisfied all stability criteria. We calculated the elastic anisotropic and the directional dependent elastic modulus of all polymorphs. Electronic properties of the involved phases were calculated by employing the hybrid functional of Heyd, Scuseria, and Erhzerhof (HSE06) and the value scattered from 3.35 to 1.86 eV. The bonding nature of the global minimum rutile phase is analyzed from charge-density, charge-transfer, electron-localization-function given in figure 1. Conclusion: Although many phases are available on SnO2, rutile is of course promising candidate for many applications [2].

• Suresh, A.; Kanimozhi, Balakrishnan; Vasu, Veerapandi & Ponniah, Vajeeston (2021). Tailoring physical properties by introducing alkali metals with di-sulphide anion for technological applications: A DFT study. Fulltekst i vitenarkiv Vis sammendrag

  Advancements in materials synthesis combined with computational materials design proved to be a powerful tool to find new materials to replace conventional materials with new exotic properties for ever growing demand of new materials for new technology. Zinc sulphide is a conventional wide bandgap semiconductor having variety of applications such as light-emitting diodes, sensors, electroluminescence and bio sensor devices. To improve its efficiency doping foreign atoms into host lattice is a one route. Recently, it is shown that multiple ions introduction in place of single ion shows remarkable modification of physical properties. In line with this, the less explored and a new series of MLiZnS2 ( M = Na, K, Rb, Cs) system is studied using first-principles method for the replacement of ZnS. We employed density functional theory (DFT) and hybrid functional of Heyd, Scuseria, and Erhzerhof (HSE06) to calculate structural, dynamical, mechanical and Raman and IR spectra for entire MLiZnS2 series.

• Chapa, Pamodani Wanniarachchi; Arunasalam, Thevakaran; Punniamoorthy, Ravirajan; Dhayalan, Velauthapillai & Ponniah, Vajeeston (2021). Mixed Anion Perovskites for photovoltaic applications – a first-principle study. Fulltekst i vitenarkiv Vis sammendrag

  In the field of photovoltaic research community of double perovskites solar cells acts a major role by considering their environmental issues and the matter of stability. Using density functional theory calculations, we study structural and electronic properties of double perovskites Cs2AgBiX6 (X = Br, Cl, F, I) as they belonged to Fm-3m (225) space groups of cubic or tetragonal phases. Structural stability and electronic structure analysis of all the compounds have been carried out using the VASP simulation package by using the exchange-correlation functional with generalized gradient approximation method (GGA) and hybrid functional (HSE06). Energy volume data are fitted with Birch-Murnaghan’s equation of state and equilibrium lattice parameters are obtained. Cs2AgBiX6 phases are stabilized in the cubic phase except for Cs2AgBiI6 and it stabilizes in the tetragonal phase. Electronic band structures and orbital-projected density of states of double perovskite materials were simulated. The calculated band gaps (Eg) were simulated using screened hybrid functional as proposed by Heyd, Scuseria and Ernzerhof (HSE06) for the polymorphs optimized at the PBE level and it is found as an indirect band gap. The calculated Eg in HSE06(GGA) for Cs2AgBiBr6, Cs2AgBiCl6, Cs2AgBiF6 and Cs2AgBiI6 are 1.97eV(1.35eV), 2.92eV(1.86eV), 4.27eV(2.75eV) and 1.21eV(0.83eV) respectively and they are in good agreement with the past theoretical and experimental investigations. We also evaluated the carrier mobilities of electron and holes along with the three directions of the first Brillouin zone G to K, G to L, and G to X using effective mass calculations for the entire series. These theoretical results show the new approach to fabricate environmental friendly double perovskites solar cells alternative to the lead halide perovskites.

• Murugesan, Rasukkannu; Dhayalan, Velauthapillai & Ponniah, Vajeeston (2021). Properties of Natural Intermediate band in Cu2CdGeS4 for Intermediate band solar cells: A First-Principle Insight. Fulltekst i vitenarkiv Vis sammendrag

  Introduction: The development of intermediate band solar cells is associated with materials with an intermediate band between the valence and conduction bands which can be used the stepping stone for generating electron-hole pair. These materials are generally produced artificially, which generates practical challenges on the synthesis side. Methods and Results: In this theoretical study, we investigate a natural intermediate band material, Cu2CdGeS4, which has the intermediate band in the main band gap. Cu2CdGeS4 is I2-II-IV-VI4 group quaternary Cu-based chalcogenide semiconductor, which has been proposed as promising high efficiency light-absorber layer. For the first time, we found the isolated conduction band of Cu2CdGeS4 in the zincblende or wurtzite structure, which can be regarded as a natural intermediate band by employing a hybrid functional model. We have demonstrated the absorption coefficient of both valance band to intermediate band and intermediate band to conduction band transition using Bethe-Salpeter equation (BSE) method. These steps of optical transitions are corresponding to electronic transitions. Conclusions: Hence, the results imply that Cu2CdGeS4 being two-photon absorption in the visible light range. Additionally, the hole and electron effective masses of Cu2CdGeS4 are systematically studied using first-principle calculations. We show that Cu2CdGeS4 could make possible intermediate band solar cells with improved efficiency.

• Lathifa, Banu; Vasu, Veerapandi & Ponniah, Vajeeston (2021). First-principles insights into the relative stability, physical and chemical properties of MoSe2. Fulltekst i vitenarkiv Vis sammendrag

  In the present work, the relative stability of different MoSe2 polymorphs are proposed and in-depth analysis of these polymorphs are carried out by employing first-principle calculations based on density functional theory (DFT). Structurally, MoSe2 can be regarded as strongly bonded two dimensional Se–Mo–Se layers or sandwiches which are loosely coupled to one another by relatively weak van der Waals-type forces. Within a single Se–Mo–Se sandwich, the Mo and Se atoms create two-dimensional hexagonal arrays. Depending on the relative alignment of the two Se-atom sheets within a single Se– Mo–Se sandwich, two distinct two-dimensional crystal polymorphs are obtained. From our simulation we observe (see Figure. 1) that the various polymorphs seem to have the same minimum energy, although with a varying range of volume. Which indicates that MoSe2 can easily be found in any of these variants and 1T and 3Tpolymorphs were only found to be metastable phases and cannot be synthesized. Electronic properties of the involved phases were calculated by employing the hybrid functional of Heyd, Scuseria, and Erhzerhof (HSE06). The mechanical and dynamical stability of all these phases also investigated by single crystal elastic constant and phonon studies, respectively.

• 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.

• Eidsvåg, Håkon; Rasukkannu, Murugesan; Velauthapillai, Dhayalan & Vajeeston, Ponniah (2021). Correction: In-depth first-principle study on novel MoS2polymorphs. RSC Advances. 11(20), s. 12188–12188. doi: 10.1039/d1ra90092g. 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
• Rasukkannu, Murugesan; Dhayalan, Velauthapillai & Vajeeston, Ponniah (2020). Properties of Natural Intermediate band in Cu2CdGeS4 for Intermediate band solar cells: A First-Principle Insight. Fulltekst i vitenarkiv
• Fjellvåg, Øystein; Armstrong, J; Krzystyniak, Matthew; Vajeeston, Ponniah & Sjåstad, Anja Olafsen (2020). Structure and dynamics of La2LiHO3. Fulltekst i vitenarkiv
• Vajeeston, Ponniah (2020). Computational materials design and characterization of battery materials. Fulltekst i vitenarkiv
• Vajeeston, Ponniah (2020). High-throughput computational materials design and characterisation for functional inorganic materials. 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
• Venkatachalapathy, Vishnukanthan; Vajeeston, Ponniah; Galeckas, Augustinas; Vines, Lasse & Kuznetsov, Andrej (2018). Direct and indirect bandgaps of Cadmium Oxide. 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; 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
• El-Kharbachi, Abdelouahab; Høgset, A.; Kim, Sangryun; Vajeeston, Ponniah; Sørby, Magnus Helgerud & Fjellvåg, Helmer [Vis alle 8 forfattere av denne artikkelen] (2018). Structural and Electrochemical Study of Mixtures of Halogenated-LiBH4 and Amorphous 0.75Li2S·0.25P2S5 for Li- Conducting Solid Electrolytes. 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.

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