Vajeeston Ponniah

• 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. doi: 10.1021/acs.chemmater.4c01104.
• 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. ISSN 2053-1591. 11(2). doi: 10.1088/2053-1591/ad1f98.
• 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
• Devamanoharan, Arthi; Venkatachalapathy, Vishnukanthan; Veerapandy, Vasu & Ponniah, Vajeeston (2023). Investigating Stable Low-Energy Gallium Oxide (Ga<inf>2</inf>O<inf>3</inf>) Polytypes: Insights into Electronic and Optical Properties from First Principles. ACS Omega. ISSN 2470-1343. doi: 10.1021/acsomega.3c10192.
• 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.
• 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.
• 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. ISSN 2470-1343. 8(45), s. 42275–42289. doi: 10.1021/acsomega.3c04329. Fulltekst i vitenarkiv
• Pappathi, Murugan; Ponniah, Vajeeston; Sahaya Raja, Antony Samy; 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. ISSN 2574-0970. 6(16), s. 14628–14644. doi: 10.1021/acsanm.3c01489.
• 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
• Eidsvåg, Håkon; Ponniah, Vajeeston & Velauthapillai, Dhayalan (2023). Doped MoS<inf>2</inf> Polymorph for an Improved Hydrogen Evolution Reaction. ACS Omega. ISSN 2470-1343. 8(29), s. 26263–26275. doi: 10.1021/acsomega.3c02623. Fulltekst i vitenarkiv
• Banu S, Lathifa; Veerapandy, Vasu; Fjellvåg, Helmer & Ponniah, Vajeeston (2023). First-Principles Insights into the Relative Stability and Physical and Chemical Properties of MoSe2. ACS Omega. ISSN 2470-1343. 8(15), s. 13799–13812. doi: 10.1021/acsomega.2c08217. Fulltekst i vitenarkiv
• 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. ISSN 2470-1343. 8(14), s. 12928–12943. doi: 10.1021/acsomega.3c00105. Fulltekst i vitenarkiv
• 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.
• 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
• Sottmann, Jonas; Ruud, Amund; Fjellvåg, Øystein; Vaughan, Gavin B. M.; Di Michel, Marco & 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
• 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.
• 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. ISSN 2046-2069. 12(27), s. 17410–17421. doi: 10.1039/d2ra01834a. Fulltekst i vitenarkiv
• Devamanoharan, Arthi; Veerapandy, Vasu & Ponniah, Vajeeston (2022). Computational study on electronic and thermal stability of low energy indium oxide polytypes. Materials Today: Proceedings. ISSN 2214-7853. 64, s. 1803–1807. doi: 10.1016/j.matpr.2022.06.078. Fulltekst i vitenarkiv
• Lathifa Banu, Banu; Veerapandy, Vasu & Ponniah, Vajeeston (2022). Structural stability and electronical properties of MoSe<inf>2</inf> polymorphs: DFT. Materials Today: Proceedings. ISSN 2214-7853. 64, s. 1808–1812. doi: 10.1016/j.matpr.2022.06.079.
• 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. ISSN 2214-7853. 64, s. 1798–1802. doi: 10.1016/j.matpr.2022.06.075.
• 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.
• 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. ISSN 2214-7853. 64(Part 5), s. 1783–1788. doi: 10.1016/j.matpr.2022.06.063.
• 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. ISSN 2470-1343. 7(12), s. 10382–10393. doi: 10.1021/acsomega.1c07063.
• 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
• 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. 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.

• 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.

• 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
• 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. ISSN 2470-1343. 6(45), s. 30419–30431. doi: 10.1021/acsomega.1c03721. Fulltekst i vitenarkiv
• 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.
• Kalaiselvan, Ganesan; Govindaraj, Lingannan; Kannan, Murugesan; Christopher S., Perreault; Gopi K., Samudrala & Pankaj Kumar, Maheshwari [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
• Govindaraj, Lingannan; Boby, Joseph; Vajeeston, Ponniah; Chia Nung, Kuo; Chin Shan, Lue & 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

• 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.
• 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
• Eidsvåg, Håkon; Rasukkannu, Murugesan; Velauthapillai, Dhayalan & Vajeeston, Ponniah (2021). In-depth first-principle study on novel MoS2 polymorphs. RSC Advances. ISSN 2046-2069. 11(6), s. 3759–3769. doi: 10.1039/d0ra10443d. Fulltekst i vitenarkiv
• 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
• 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.

• 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
• 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
• 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
• 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.
• 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.
• 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. ISSN 2399-6528. 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.

• Rasukkannu, Murugesan; Velauthapillai, Dhayalan & Vajeeston, Ponniah (2019). First-Principle Calculation of High Absorption-TlGaTe2 for Photovoltaic Application. Materials. ISSN 1996-1944. 12(7). doi: 10.3390/ma12172667. Fulltekst i vitenarkiv
• 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. ISSN 1996-1073. 12(224), s. 1–24. doi: 10.3390/en12020224. Fulltekst i vitenarkiv
• 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.
• 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. 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.

• 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 (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
• Rasukkannu, Murugesan; Velauthapillai, Dhayalan & Vajeeston, Ponniah (2018). Hybrid Density Functional Study of Au2Cs2I6, Ag2GeBaS4, Ag2ZnSnS4, and AgCuPO4 for the Intermediate Band Solar Cells. Energies. ISSN 1996-1073. 11(12). doi: 10.3390/en11123457. 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. ISSN 1996-1944. 11:2006(10), s. 1–17. doi: 10.3390/ma11102006. 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
• 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.
• 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
• 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.
• 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.
• 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
• 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.
• 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.
• 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.

• 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. ISSN 1948-7185. 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.

• Sottmann, Jonas; Di Michiel, Marco; Fjellvåg, Helmer; Malavasi, Lorenzo; Margadonna, Serena & Vajeeston, Ponniah [Vis alle 8 forfattere av denne artikkelen] (2017). Chemical Structures of Specific Sodium Ion Battery Components Determined by Operando Pair Distribution Function and X-ray Diffraction Computed Tomography. Angewandte Chemie International Edition. ISSN 1433-7851. 56(38), s. 11385–11389. doi: 10.1002/anie.201704271. Fulltekst i vitenarkiv
• Bianchini, Federico; Fjellvåg, Helmer & Vajeeston, Ponniah (2017). First-principles study of the structural stability and electrochemical properties of Na2MSiO4 (M = Mn, Fe, Co and Ni) polymorphs. Physical Chemistry, Chemical Physics - PCCP. ISSN 1463-9076. 19(22), s. 14462–14470. doi: 10.1039/c7cp01395g. Fulltekst i vitenarkiv
• Vajeeston, Ponniah & Fjellvåg, Helmer (2017). First-principles study of structural stability, dynamical and mechanical properties of Li2FeSiO4 polymorphs. RSC Advances. ISSN 2046-2069. 7(27), s. 16843–16853. doi: 10.1039/c6ra26555c. Fulltekst i vitenarkiv
• Lundvall, Fredrik; Wragg, David; Vajeeston, Ponniah; Dietzel, Pascal D.C. & Fjellvåg, Helmer (2017). Ab initio structure solution and thermal stability evaluation of a new Ca(II) 3D coordination polymer using synchrotron powder X-ray diffraction data. CrysteEngComm. ISSN 1466-8033. 19(39), s. 5857–5863. doi: 10.1039/c7ce01389b. Vis sammendrag

  A new coordination polymer was synthesized hydrothermally using 4,4′-dimethoxy-3,3′-biphenyldicarboxylic acid (H2dmbpdc) and Ca(NO3)2·4H2O. The structure of the new compound, Ca(dmbpdc)(H2O)2 (CPO-70-Ca), was determined from powder X-ray diffraction (PXRD) data using simulated annealing, followed by Rietveld refinement of the structure. The framework is based on one-dimensional metal carboxylate chains that are linked by the organic linker into a three-dimensional network with rhombic channels. The as-synthesized material resembles our recently reported compound CPO-69-Ca, and shares the same sra topology. The asymmetric unit comprises one calcium atom, one fully deprotonated linker, one water molecule coordinated to the metal cation as well as one solvent water molecule located in the rhombic channels. The metal cation is eight-coordinate, forming an irregular coordination polyhedron. The thermal behavior of CPO-70-Ca was investigated in an in situ structural analysis by variable temperature powder X-ray diffraction. This study revealed an interesting response to solvent removal from the as-synthesized structure. The results include a partially desolvated version of CPO-70-Ca and a structural transformation to CPO-69-Ca upon full desolvation.

• Nagell, Marius Uv; Slawinski, Wojciech Andrzej; Vajeeston, Ponniah; Fjellvåg, Helmer & Sjåstad, Anja Olafsen (2017). Temperature induced transitions in La-4(Co1-xNix)(3)O-10 (+) (delta); oxygen stoichiometry and mobility. Solid State Ionics. ISSN 0167-2738. 305, s. 7–15. doi: 10.1016/j.ssi.2017.04.006.

Se alle arbeider i Cristin

• Sottmann, Jonas; Ruud, Amund; Fjellvåg, Øystein Slagtern; Vaughan, Gavin B. M.; Di Michel, Marco & 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.
• 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. 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. 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). 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.
• 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. 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; 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. 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.

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

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

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

• 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. . 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.

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

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

• 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. 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. ISSN 2046-2069. 11(20), s. 12188–12188. doi: 10.1039/d1ra90092g.
• 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.
• 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.
• Vajeeston, Ponniah (2020). Computational materials design and characterization of battery materials.
• Vajeeston, Ponniah (2020). High-throughput computational materials design and characterisation for functional inorganic materials.
• Rasukkannu, Murugesan; Dhayalan, Velauthapillai & Vajeeston, Ponniah (2020). Properties of Natural Intermediate band in Cu2CdGeS4 for Intermediate band solar cells: A First-Principle Insight.
• Fjellvåg, Øystein; Armstrong, J; Krzystyniak, Matthew; Vajeeston, Ponniah & Sjåstad, Anja Olafsen (2020). Structure and dynamics of La2LiHO3.
• 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.
• Wragg, David; Ruud, Amund; Fjellvåg, Øystein; Fjellvåg, Helmer; lebedev, Oleg & Di Michel, Marco [Vis alle 9 forfattere av denne artikkelen] (2019). PDF Analysis of Batteries Using Diffraction Computed Tomography.
• 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.
• 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.
• 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.
• 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.
• Venkatachalapathy, Vishnukanthan; Vajeeston, Ponniah; Galeckas, Augustinas; Vines, Lasse & Kuznetsov, Andrej (2018). Direct and indirect bandgaps of Cadmium Oxide.
• Vajeeston, Ponniah & Fjellvåg, Helmer (2017). Computational materials design and characterisation for energy harvesting and energy storage materials.
• Vajeeston, Ponniah (2017). Computational materials design for energy harvesting and energy storage materials.
• Venkatachalapathy, Vishnukanthan; Vajeeston, Ponniah; Vines, Lasse; Monakhov, Edouard & Kuznetsov, Andrej (2017). Ion irradiation induced phase transformation in crystalline Si.
• Venkatachalapathy, Vishnukanthan; Ayedh, Hussein Mohammed Hussein; Vajeeston, Ponniah; Vines, Lasse; Monakhov, Edouard & Kuznetsov, Andrej (2017). Quest for Si based multi-junction solar cells covering the entire solar spectrum.
• Wragg, David; Sottmann, Jonas; Vajeeston, Ponniah & Ruud, Amund (2017). The intriguing mechanism of phosphorus anodes for sodium ion batteries Revealed by operando pair distribution function and X-ray diffraction computed tomography.
• Hu, Yang; El-Kharbachi, Abdelouahab; Yoshida, Koji; Vajeeston, Ponniah; Sørby, Magnus Helgerud & Orimo, Shin-Ichi [Vis alle 8 forfattere av denne artikkelen] (2017). Ionic Properties of the borohydride-sulfide Mixed System for All-Solid Li-Ion Batteries.
• 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.

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