Ponniah Ravindran

Bio: Ponniah Ravindran is an academic researcher from Central University of Tamil Nadu. The author has contributed to research in topics: Density functional theory & Band gap. The author has an hindex of 44, co-authored 202 publications receiving 7631 citations. Previous affiliations of Ponniah Ravindran include University of Bristol & University of Padua.

Density functional theory for calculation of elastic properties of orthorhombic crystals: Application to TiSi2

Abstract: A theoretical formalism to calculate the single crystal elastic constants for orthorhombic crystals from first principle calculations is described. This is applied for TiSi2 and we calculate the elastic constants using a full potential linear muffin-tin orbital method using the local density approximation (LDA) and generalized gradient approximation (GGA). The calculated values compare favorably with recent experimental results. An expression to calculate the bulk modulus along crystallographic axes of single crystals, using elastic constants, has been derived. From this the calculated linear bulk moduli are found to be in good agreement with the experiments. The shear modulus, Young’s modulus, and Poisson’s ratio for ideal polycrystalline TiSi2 are also calculated and compared with corresponding experimental values. The directional bulk modulus and the Young’s modulus for single crystal TiSi2 are estimated from the elastic constants obtained from LDA as well as GGA calculations and are compared with the ...

Theoretical investigation of magnetoelectric behavior in Bi Fe O 3

Abstract: The magnetoelectric behavior of $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$ has been explored on the basis of accurate density functional calculations. We are able to predict structural, electronic, magnetic, and ferroelectric properties of $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$ correctly without including any strong correlation effect in the calculation. Unlike earlier calculations, the equilibrium structural parameters are found to be in very good agreement with the experimental findings. In particular, the present calculation correctly reproduced experimentally observed elongation of cubic perovskitelike lattice along the [111] direction. At high pressure we predicted a pressure-induced structural transition from rhombohedral $(R3c)$ to an orthorhombic $(Pnma)$ structure. The total-energy calculations at expanded lattice show two lower energy ferroelectric phases (with monoclinic $Cm$ and tetragonal $P4mm$ structures), closer in energy to the ground-state phase. Spin-polarized band-structure calculations show that $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$ will be an insulator in $A$- and $G$-type antiferromagnetic phases and a metal in $C$-type antiferromagnetic, ferromagnetic configurations, and in the nonmagnetic state. Chemical bonding in $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$ has been analyzed using partial density of states, charge density, charge transfer, electron localization function, Born-effective-charge tensor, and crystal orbital Hamiltonian population analyses. Our electron localization function analysis shows that stereochemically active lone-pair electrons are present at the Bi sites which are responsible for displacements of the Bi atoms from the centrosymmetric to the noncentrosymmetric structure and hence the ferroelectricity. A large ferroelectric polarization of $88.7\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{C}∕{\mathrm{cm}}^{2}$ is predicted in accordance with recent experimental findings, but differing by an order of magnitude from earlier experimental values. The strong spontaneous polarization is related to the large values of the Born-effective charges at the Bi sites along with their large displacement along the [111] direction of the cubic perovskite-type reference structure. Our polarization analysis shows that partial contributions to polarization from the Fe and O atoms almost cancel each other and the net polarization present in $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$ mainly $(g98%)$ originates from Bi atoms. We found that the large scatter in experimentally reported polarization values in $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$ is associated with the large anisotropy in the spontaneous polarization.

Electronic structure, bonding, and ground-state properties of AlB 2 -type transition-metal diborides

Abstract: The electronic structure and ground state properties of ${\mathrm{AlB}}_{2}$ type transition metal diborides ${\mathrm{TMB}}_{2}$ (TM=Sc, Ti, V, Cr, Mn, Fe, Y, Zr, Nb, Mo, Hf, Ta) have been calculated using the self consistent tight-binding linear muffin-tin orbital method. The equilibrium volume, bulk moduli ${(B}_{0}),$ pressure derivative of bulk moduli ${(B}_{0}^{\ensuremath{'}}),$ cohesive energy ${(E}_{\mathrm{coh}}),$ heat of formation $(\ensuremath{\Delta}H),$ and electronic specific heat coefficient $(\ensuremath{\gamma})$ are calculated for these systems and compared with the available experimental and other theoretical results. The bonding nature of these diborides is analyzed via the density of states (DOS) histogram as well as the charge density plots, and the chemical stability is analyzed using the band filling principle. The variation in the calculated cohesive properties of these materials is correlated with the band filling effect. The existence of a pseudogap in the total density of states is found to be a common feature for all these compounds. The reason for the creation of the pseudogap is found to be due to the strong covalent interaction between boron p states. We have made spin polarized calculations for ${\mathrm{CrB}}_{2},$ ${\mathrm{MnB}}_{2},$ and ${\mathrm{FeB}}_{2}$ and found that finite magnetic moments exist for ${\mathrm{MnB}}_{2}$ and ${\mathrm{CrB}}_{2}$ whereas ${\mathrm{FeB}}_{2}$ is nonmagnetic.

Electronic structure and optical properties of ZnX ( X=O, S, Se, Te): A density functional study

Abstract: Electronic band structure and optical properties of zinc monochalcogenides with zinc-blende- and wurtzite-type structures were studied using the ab initio density functional method within the local-density approximation (LDA), generalized-gradient approximation, and $\mathrm{LDA}+U$ approaches. Calculations of the optical spectra have been performed for the energy range $0--20\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, with and without including spin-orbit coupling. Reflectivity, absorption and extinction coefficients, and refractive index have been computed from the imaginary part of the dielectric function using the Kramers-Kronig transformations. A rigid shift of the calculated optical spectra is found to provide a good first approximation to reproduce experimental observations for almost all the zinc monochalcogenide phases considered. By inspection of the calculated and experimentally determined band-gap values for the zinc monochalcogenide series, the band gap of ZnO with zinc-blende structure has been estimated.

Phase stability, electronic structure, and optical properties of indium oxide polytypes

Abstract: ¯ are indirect band gap semiconductors, while the other phase of space group Ia3 is having direct band gap. The calculated carrier effective masses for all these three phases are compared with available experimental and theoretical values. From charge-density and electron localization function analysis, it is found that these phases have dominant ionic bonding with noticeable covalent interaction between indium and oxygen. The magnitudes of the absorption and reflection coefficients for In2O3 with space groups Ia3 and R3 are small in the energy range 0 – 5 eV, indicating that these phases can be regarded and classified as transparent.

Physics and Applications of Bismuth Ferrite

Abstract: BiFeO3 is perhaps the only material that is both magnetic and a strong ferroelectric at room temperature. As a result, it has had an impact on the field of multiferroics that is comparable to that of yttrium barium copper oxide (YBCO) on superconductors, with hundreds of publications devoted to it in the past few years. In this Review, we try to summarize both the basic physics and unresolved aspects of BiFeO3 (which are still being discovered with several new phase transitions reported in the past few months) and device applications, which center on spintronics and memory devices that can be addressed both electrically and magnetically.

Ferromagnetic materials

Abstract: In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Complex hydrides for hydrogen storage.

Abstract: Note: Times Cited: 875 Reference EPFL-ARTICLE-206025doi:10.1021/cr0501846View record in Web of Science URL: ://WOS:000249839900009 Record created on 2015-03-03, modified on 2017-05-12

Complex Hydrides for Hydrogen Storage

Abstract: Note: Times Cited: 875 Reference EPFL-ARTICLE-206025doi:10.1021/cr0501846View record in Web of Science URL: ://WOS:000249839900009 Record created on 2015-03-03, modified on 2017-05-12

Refractory Diborides of Zirconium and Hafnium

Abstract: This paper reviews the crystal chemistry, synthesis, densification, microstructure, mechanical properties, and oxidation behavior of zirconium diboride (ZrB2) and hafnium diboride (HfB2) ceramics. The refractory diborides exhibit partial or complete solid solution with other transition metal diborides, which allows compositional tailoring of properties such as thermal expansion coefficient and hardness. Carbothermal reduction is the typical synthesis route, but reactive processes, solution methods, and pre-ceramic polymers can also be used. Typically, diborides are densified by hot pressing, but recently solid state and liquid phase sintering routes have been developed. Fine-grained ZrB2 and HfB2 have strengths of a few hundred MPa, which can increase to over 1 GPa with the addition of SiC. Pure diborides exhibit parabolic oxidation kinetics at temperatures below 1100°C, but B2O3 volatility leads to rapid, linear oxidation kinetics above that temperature. The addition of silica scale formers such as SiC or MoSi2 improves the oxidation behavior above 1100°C. Based on their unique combination of properties, ZrB2 and HfB2 ceramics are candidates for use in the extreme environments associated with hypersonic flight, atmospheric re-entry, and rocket propulsion.