Vidya Ravindran - Academia.edu (original) (raw)
Papers by Vidya Ravindran
Variations in the spin, charge, and orbital ordering are analyzed in terms of the d-band filling ... more Variations in the spin, charge, and orbital ordering are analyzed in terms of the d-band filling for YBaT2O5 (T = Mn, Fe, Co). The calculations are performed with accurate density-functional-theory methods as implemented in the full-potential linearized-augmented plane-wave approach. We have carried out calculations for paramagnetic, ferromagnetic, and antiferromagnetic configurations. A ferrimagnetic ground state has been established for YBaMn2O5 whereas YBaFe2O5 and YBaCo2O5 have antiferromagnetic ground states in agreement with experimental findings. The electronic band characteristics are analyzed using total, site-, and orbital-projected density of states. Inclusion of spin-orbit coupling and Coulomb correlation effects are found to be important for YBaFe2O5 and YBaCo2O5 in order to reproduce the experimentally established semiconducting behavior. The different types of charge and orbital ordering in these compounds are visualized by making use of the energy-projected density m...
ACS Omega, 2019
Giant magnetoelectric coupling is a very rare phenomenon that has gained much attention in the pa... more Giant magnetoelectric coupling is a very rare phenomenon that has gained much attention in the past few decades due to fundamental interest as well as practical applications. Here, we have successfully achieved giant magnetoelectric coupling in PbTi 1−x V x O 3 (x = 0−1) using a series of generalized gradient-corrected GGA (generalized gradient approximation), including on-site Coulomb repulsion (U)-corrected spin-polarized calculations based on accurate density functional theory. Our total energy calculations show that PbTi 1−x V x O 3 stabilizes in C-type antiferromagnetic ground state for x > 0.123. With the substitution of V into PbTiO 3 , the tetragonal distortion is highly enhanced accompanied by a linear increase in polarization. In addition, our band structure analysis shows that for lower x values, the tendency to form two-dimensional magnetism of PbTi 1−x V x O 3 decreases. The orbital magnetic polarization was calculated with self-consistent field method by including orbital polarization correction in the calculation as well as from the computed X-ray magnetic dichroism spectra. A nonmagnetic metallic ground state is observed for the paraelectric phase for V concentration (x) = 1 competing with a volume change of 10% showing a large magnetovolume effect. Our orbital-projected density of states as well as orbital ordering analysis suggest that the orbital ordering plays a major role in the magnetic-to-nonmagnetic transition when going from ferroelectric to paraelectric phase. The calculated magnetic anisotropic energy shows that the direction [110] is the easy axis of magnetization for x = 1 composition. The partial polarization analysis shows that the Ti/V−O hybridization majorly contributes to the total electrical polarization. The present study adds a new series of compounds to the magnetoelectric family with rarely existing giant coupling between electric-and magnetic-order parameters. These results show that such kind of materials can be used for novel practical applications where one can change the magnetic properties drastically (magnetic to nonmagnetic, as shown here) with external electric field and vice versa.
Materials Research Express, 2019
Finding potential materials for solar cell applications is essential to reduce cost and enhance e... more Finding potential materials for solar cell applications is essential to reduce cost and enhance efficiency. We have employed Density Functional Theory (DFT) based calculations for novel nitrides of type A3N2 (A = Mg, Zn ans Sn) and Sn3N4 to find the ground state crystal and electronic structure. The structural parameters optimized by theoretical calculation are in good agreement with the experimental parameters. In order to obtain Sn3N2 and Sn3N4 experimentally, we are suggesting a new synthesis route from the calculated enthalpy of formation. The studied nitrides exhibit semiconductor behavior with direct band gaps. Band gap of 1.7 eV is obtained for Mg3N2 from GGA calculation. For Sn3N4 band gaps of 0.24 eV and 0.7 eV are obtained using GGA and LDA calculations, respectively, whereas GGA+U was used to obtain band gap in Zn3N2. The bonding behavior is analyzed in detail by using charge density and electron localization function plots. In addition, COHP (Crystal Orbital Hamiltonian Population) is utilized to retrieve bond strength values. Charge transfer from cation to anion decreases from Mg to Sn and correspondingly bond strength between the metal and Nitrogen atoms is also found to increase from Mg to Sn, which indicates increase in covalent nature of bonding from Mg to Sn. Zn3N2 is found to have the possibility for n-type as well as p-type doping because of the low effective mass of electrons and holes compared to other studied nitrides. Hence, Zn3N2 has suitable conductive properties to be used as a solar cell material.
Physical Review B, 2002
The oxygen-deficient (double) perovskite YBaMn 2 O 5 , containing cornerlinked MnO 5 square pyram... more The oxygen-deficient (double) perovskite YBaMn 2 O 5 , containing cornerlinked MnO 5 square pyramids, is found to exhibit ferrimagnetic ordering in its ground state. In the present work we report generalized-gradient-corrected, relativistic first-principles full-potential density-functional calculations performed on YBaMn 2 O 5 in the nonmagnetic, ferromagnetic and ferrimagnetic states. The charge, orbital and spin orderings are explained with site-, angular momentum-and orbital-projected density of states, charge-density plots, electronic structure and total energy studies. YBaMn 2 O 5 is found to stabilize in a G-type ferrimagnetic state in accordance with experimental results. The experimentally observed insulating behavior appears only when we include ferrimagnetic ordering in our calculation. We observed significant optical anisotropy in this material originating from the combined effect of ferrimagnetic ordering and crystal field splitting. In order to gain knowledge about the presence of different valence states for Mn in YBaMn 2 O 5 we have calculated K-edge x-ray absorption near-edge spectra for the Mn and O atoms. The presence of the different valence states for Mn is clearly established from the x-ray absorption near-edge spectra, hyperfine field parameters and the magnetic properties study. Among the experimentally proposed structures, the recently reported description based on P 4/nmm is found to represent the stable structure.
Physical Review B, 2003
A detailed high-pressure study on LiAlH 4 has been carried out using the ab initio projected augm... more A detailed high-pressure study on LiAlH 4 has been carried out using the ab initio projected augmented plane-wave method. Application of pressure transforms ␣to -LiAlH 4 (␣-NaAlH 4-type structure͒ at 2.6 GPa with a huge volume collapse of 17%. This abnormal behavior is associated with electronic transition from Al-s top states. At 33.8 GPa, a  to ␥ transition is predicted from ␣-NaAlH 4-type to KGaH 4-type structure. Up to 40 GPa LiAlH 4 remains nonmetallic. The high weight percent of hydrogen, around 22% smaller equilibrium volume, and drastically different bonding behavior than ␣-phase indicate that -LiAlH 4 is expected to be a potential hydrogen storage material.
Physical Review B, 2003
First-principle studies on the total energy, electronic structure, and bonding nature of RNiIn (R... more First-principle studies on the total energy, electronic structure, and bonding nature of RNiIn (RϭLa, Ce, and Nd͒, and their saturated hydrides (R 3 Ni 3 In 3 H 4 ϭRNiInH 1.333) are performed using a full-potential linear muffin-tin orbital approach. This series of phases crystallizes in a ZrNiAl-type structural framework. When hydrogen is introduced in the RNiIn matrix, anisotropic lattice expansion is observed along ͓001͔ and lattice contraction along ͓100͔. In order to establish the equilibrium structural parameters for these compounds we have performed force minimization as well as volume and c/a optimization. The optimized atomic positions, cell volume, and c/a ratio are in very good agreement with recent experimental findings. From the electronic structure and charge density, charge difference, and electron localization function analyses the microscopic origin of the anisotropic change in lattice parameters on hydrogenation of RNiIn has been identified. The hydrides concerned, with their theoretically calculated interatomic H-H distances of ϳ1.57 Å, violate the ''2-Å rule'' for H-H separation in metal hydrides. The shortest internuclear Ni-H separation is almost equal to the sum of the covalent radii. H is bonded to Ni in an H-Ni-H dumbbell-shaped linear array, with a character of NiH 2 subunits. Density of states, valence charge density, charge transfer plot, and electron localization function analyses clearly indicate significant ionic bonding between Ni and H and weak metallic bonding between H-H. The paired and localized electron distribution at the H site is polarized toward La and In which reduces the repulsive interaction between negatively charged H atoms. This could explain the unusually short H-H separation in these materials. The calculations show that all these materials have a metallic character.
Physical Review B, 2001
In order to understand the unexpected superconducting behavior of MgB 2 we have made electronic s... more In order to understand the unexpected superconducting behavior of MgB 2 we have made electronic structure calculations for MgB 2 and closely related systems. Our calculated Debye temperature from the elastic properties indicate that the average phonon frequency is very large in MgB 2 compared with other superconducting intermetallics and its exceptionally higher T c can be explained through a BCS mechanism only if phonon softening occurs or the phonon modes are highly anisotropic. We identified a doubly degenerate quasi-twodimensional key-energy band in the vicinity of E F along the ⌫-A direction of BZ ͑having equal amount of B p x and p y character͒ which plays an important role in deciding the superconducting behavior. Based on this result, we have searched for a similar electronic feature in isoelectronic compounds such as BeB 2 , CaB 2 , SrB 2 , LiBC, and MgB 2 C 2 and found that hole doped LiBC and MgB 2 C 2 are potential superconducting materials. We have found that E F in the closely related compound MgB 4 is lying in a pseudogap with a negligibly small density of states at E F , which is not favorable for superconductivity. There are contradictory experimental results regarding the anisotropy in the elastic properties of MgB 2 ranging from isotropic to moderately anisotropic to highly anisotropic. In order to settle this issue we have calculated the single-crystal elastic constants for MgB 2 by the accurate full-potential method and derived the directional-dependent linear compressibility, Young's modulus, shear modulus, and relevant elastic properties from these results. We have observed large anisotropy in the elastic properties consistent with recent high-pressure findings. Our calculated polarized optical dielectric tensor shows highly anisotropic behavior even though it possesses isotropic transport property. MgB 2 possesses a mixed bonding character and this has been verified from density of states, charge density, and crystal orbital Hamiltonian population analyses.
Solid State Communications, 2002
We have made a series of gradient-corrected relativistic full-potential density-functional calcul... more We have made a series of gradient-corrected relativistic full-potential density-functional calculation for Ca-substituted and hole-doped SrRuO3 in para, ferro, and A-, C-, and G-type antiferromagnetic states. Magnetic phase-diagram data for Sr1−xCaxRuO3 at 0 K are presented. Neutron diffraction measurement combined with total energy calculations show that spin-glass behavior with short-range antiferromagnetic interactions rules in CaRuO3. The substitution of Sr by Ca in SrRuO3 decreases the ferromagnetic interaction and enhances the G-type antiferromagnetic interaction; the G-AF state is found to stabilize around x = 0.75 consistent with experimental observations. Inclusion of spin-orbit coupling is found to be important in order to arrive at the correct magnetic ground state in ruthenates.
Physical Review Letters, 2002
Using gradient-corrected, all-electron, full-potential, density-functional calculations, includin... more Using gradient-corrected, all-electron, full-potential, density-functional calculations, including structural relaxations, it is found that the metal hydrides RT InH1.333 (R = La, Ce, Pr, or Nd; T = Ni, Pd, or Pt) possess unusually short H−H separations. The most extreme value (1.454Å) ever obtained for metal hydrides occurs for LaPtInH1.333. This finding violates the empirical rule for metal hydrides, which states that the minimum H−H separation is 2Å. Electronic structure, charge density, charge transfer, and electron localization function analyses on RT InH1.333 show dominant metallic bonding with a non-negligible ionic component between T and H, the H−H interaction being weakly metallic. The paired, localized, and bosonic nature of the electron distribution at the H site are polarized towards La and In which reduces the repulsive interaction between negatively charged H atoms. This could explain the unusually short H−H separation in these materials. Also, R−R interactions contribute to shielding of the repulsive interactions between the H atoms.
Physical Review B, 2006
Accurate ab initio density-functional calculations are performed to predict ground-state crystal ... more Accurate ab initio density-functional calculations are performed to predict ground-state crystal structures and to gain understanding of electronic structure and magnetic properties of CsCr 3 O 8 and ACr 3 O 8 ͑A =In,Tl,Cu,Ag,Au͒. CsCr 3 O 8 stabilizes in an orthorhombic ͑prototype; Pnma͒ structure in agreement with experimental findings whereas the remaining compounds stabilize in the monoclinic KCr 3 O 8-type ͑C2/m͒ structure. All compounds exhibit antiferromagnetic ordering in the ground state at 0 K. The electronic structures are analyzed with the help of density-of-states, charge-density, and electron-localization-function plots. All compounds ͑except InCr 3 O 8 ͒ are found to be semiconductors ͑insulators at 0 K͒ with very small band gaps, and Cr atoms in different environments consistently take different valence states.
Physical Review B, 2004
Using generalized-gradient-corrected full-potential density-functional calculations we have studi... more Using generalized-gradient-corrected full-potential density-functional calculations we have studied the magneto-optical properties of double perovskites A 2 BBЈO 6 , A = Ca, Sr, and Ba; B = Fe; and BЈ = Mo, W, and Re. Sr 2 FeWO 6 has the maximum polar Kerr rotation of 3.87°and specific Faraday rotation of 4.5 ϫ 10 5 deg cm −1. All other compounds have Kerr rotation more than 1°, except Ca 2 FeMoO 6 which has maximum Kerr rotation of 0.5°. Our electronic structure studies show that all these compounds are half-metallic in the ferromagnetic configuration. The large Kerr rotation is found to be due to the combined effects of relatively large exchange splitting from Fe, large spin-orbit coupling due to 4d or 5d elements, large offdiagonal conductivity, and half-metallicity.
Physical Review B, 2008
The structural, electronic, and magnetic properties of the oxygen-vacancy-ordered mixed-valence S... more The structural, electronic, and magnetic properties of the oxygen-vacancy-ordered mixed-valence Sr 4 Fe 4 O 11 phase have been investigated using spin-polarized electronic-structure total-energy calculations. The optimized structural parameters obtained from accurate total-energy calculations are found to be in very good agreement with low-temperature neutron-diffraction findings. Among the different spin configurations considered for Sr 4 Fe 4 O 11 , the G-type antiferromagnetic configuration is found to represent the magnetic ground state. The calculations show finite magnetic moments at both Fe sites, and this is against the conclusions arrived at from Mössbauer spectroscopy and low-temperature powder neutron-diffraction measurements, but consistent with the results from magnetization measurements. The present study clearly show that one of the magnetic sublattices is frustrated and hence Sr 4 Fe 4 O 11 can be considered as a phase-separated system with one phase in the G-type antiferromagnetic state and the other in a spin-glass-like state. Our theoretical results show unambiguously that Fe atoms with the square pyramidal environment have a lower oxidation state than that in the octahedral coordination. However, the presence of a covalent interaction between Fe and the neighboring oxygen atoms makes the actual oxidation state considerably smaller than the formal oxidation state of 3+ and 4+ for square-pyramidal and octahedral coordination, respectively. Sr 4 Fe 4 O 11 is found to be semiconducting in the antiferromagnetic ground state.
Physical Review B, 2006
Ground-state crystal structures are predicted for Cr 3 O 8 and LiCr 3 O 8 from accurate first-pri... more Ground-state crystal structures are predicted for Cr 3 O 8 and LiCr 3 O 8 from accurate first-principles densityfunctional calculations by considering several ͑25 for the former and 12 for the latter phase͒ related structure types. Both phases stabilize in monoclinic ͑C2/m͒ structures with slight variations in the atomic arrangement of the chromium-oxygen framework. Structural optimization is also performed for Cr 8 O 21 , the obtained structural parameters being in good agreement with the experimental values. A possible synthesis route for Cr 3 O 8 is suggested from total energy studies. Electronic structure studies indicate insulating behavior with very small band gaps for Cr 8 O 21 and LiCr 3 O 8 whereas Cr 3 O 8 has pseudogaplike feature. Magnetic property and partial density of states analyses indicate two different valence states for Cr in all studied phases ͑except CrO 3 ͒. Bonding characteristics and effects of Li intercalation in Cr 3 O 8 are also analyzed with the help of charge density and charge transfer plots.
Physical Review B, 2008
From analyses of structural information for oxides with Fe in different oxidation states and comp... more From analyses of structural information for oxides with Fe in different oxidation states and computationally estimated Mössbauer parameters ͑hyperfine field, isomer shift, and quadrupole splitting͒ based on densityfunctional theory, we show that the charges residing on the different constituents cannot be directly derived either from structural or Mössbauer measurements. We have analyzed charge density, charge transfer, electron localization function, crystal orbital Hamilton population, and partial density of states to explain the bonding characteristics. Born-effective charge tensor is used to quantify the charges present at the atomic sites in Sr 4 Fe 4 O 11. We show that the effects of covalence are important in explaining the electronic structure, magnetism, and chemical bonding in oxygen-vacancy-ordered systems such as Sr 4 Fe 4 O 11 and on ignoring covalence, one can be misled in oxidation-state assignments.
Journal of Solid State Chemistry, 2003
The development in theoretical condensed-matter science based on density-functional theory (DFT) ... more The development in theoretical condensed-matter science based on density-functional theory (DFT) has reached a level where it is possible, from ''parameter-free'' quantum mechanical calculations to obtain total energies, forces, vibrational frequencies, magnetic moments, mechanical and optical properties and so forth. The calculation of such properties are important in the analyses of experimental data and they can be predicted with a precision that is sufficient for comparison with experiments. It is almost impossible to do justice to all developments achieved by DFT because of its rapid growth. Hence, it has here been focused on a few advances, primarily from our laboratory. Unusual bonding behaviors in complex materials are conveniently explored using the combination of charge density, charge transfer, and electron-localization function along with crystal-orbital Hamilton-population analyses. It is indicated that the elastic properties of materials can reliably be predicted from DFT calculations if one takes into account the structural relaxations along with gradient corrections in the calculations. Experimental techniques have their limitations in studies of the structural stability and pressure-induced structural transitions in hydride materials whereas the present theoretical approach can be applied to reliably predict properties under extreme pressures. From the spin-polarized, relativistic full-potential calculations one can study novel materials such as ruthenates, quasi-one-dimensional oxides, and spin-, charge-, and orbitalordering in magnetic perovskite-like oxides. The importance of orbital-polarization correction to the DFT to predict the magnetic anisotropy in transition-metal compounds and magnetic moments in lanthanides and actinides are emphasized. Apart from the fullpotential treatment, proper magnetic ordering as well as structural distortions have to be taken into account to predict correctly the insulating behavior of transition-metal oxides. The computational variants LDA and GGA fail to predict insulating behavior of Mott insulators whereas electronic structures can be described correctly when correlation effects are taken into account through LDA þ U or similar approaches to explain their electronic structures correctly. Excited-state properties such as linear optical properties, magneto-optical properties, XANES, XPS, UPS, BIS, and Raman spectra can be obtained from accurate DFT calculations.
Journal of Crystal Growth, 2004
The role of structural distortion, magnetic ordering, and Coulomb-correlation effect on the elect... more The role of structural distortion, magnetic ordering, and Coulomb-correlation effect on the electronic structure of perovskite-like oxides is analyzed. The density-functional theory (DFT) is originally devised to describe the groundstate properties of materials. However, during recent years it has been seen that the DFT can also be used to study excited state properties succesfully. We have recently calculated the electronic structure and linear optical properties for the series LaXO 3 (X ¼ Sc-Cu) and found that the gradient-corrected DFT describes correctly the insulating behavior of the ionic insulator LaScO 3 and the charge-transfer insulators LaCrO 3 ; LaFeO 3 ; and LaMnO 3 although the band gaps are systematically underestimated. For example, the good agreement between experimental and theoretical reflectivity spectra for LaCrO 3 clearly demonstrates that accurate full-potential DFT calculations not only describe the occupied and unoccupied states of the bands well, but also reproduce their characters. We have also calculated XPS, XANES, and magneto-optical spectra for perovskite-like oxides. For Mott-Hubbard insulators such as LaTiO 3 and LaVO 3 the DFT failed to predict insulating behavior and here the LDA+U method is applied to describe the electronic structure correctly.
Journal of Applied Physics, 2012
First-principles density functional calculations have been performed on Li-doped ZnO using allele... more First-principles density functional calculations have been performed on Li-doped ZnO using allelectron projector augmented plane wave method. Li was considered at six different interstitial sites (Li i), including anti-bonding and bond-center sites and also in substitutional sites such as at Zn-site (Li zn) and at oxygen site (Li o) in the ZnO matrix. Stability of Li Zn over Li i is shown to depend on synthetic condition, viz., Li Zn is found to be more stable than Li i under O-rich conditions. Hybrid density functional calculations performed on Li Zn indicate that it is a deep acceptor with (0/-) transition taking place at 0.74 eV above valence band maximum. The local vibrational frequencies for Li-dopants are calculated and compared with reported values. In addition, we considered the formation of Li-pair complexes and their role on electronic properties of ZnO. Present study suggests that at extreme oxygen-rich synthesis condition, a pair of acceptor type Li Zn-complex is found to be stable over the compensating Li i þ Li Zn pair. The stability of complexes formed between Li impurities and various intrinsic defects is also investigated and their role on electronic properties of ZnO has been analyzed. We have shown that a complex between Li Zn and oxygen vacancy has less formation energy and donor-type character and could compensate the holes generated by Li-doping in ZnO. V
Europhysics Letters (EPL), 2005
To design new hydrogen storage materials a main prerequisite is the knowledge about the site pref... more To design new hydrogen storage materials a main prerequisite is the knowledge about the site preference of hydrogen in the corresponding nonhydride phases. From the systematic investigation of ZrNiAl-type phases we found that the electron-localization function (ELF) can correctly predict the site preference of hydrogen in metallic phases. From the site preference trends of H in ZrNiAl-type phases we have shown that hydrogen prefers to occupy interstitial sites where ELF indicates a maximum value. Based on this observation we generalized an empirical site-preference rule which states that hydrogen prefers to occupy the interstitial sites where electrons have relatively more nonbonding localized nature than other possible sites in metals, alloys and intermetallic frameworks.
Crystal Growth & Design, 2004
The crystal and electronic structures of the entire series of alkali aluminum and alkali gallium ... more The crystal and electronic structures of the entire series of alkali aluminum and alkali gallium tetrahydrides (ABH 4 ; A) Li, Na, K, Rb, or Cs; B) Al or Ga) are systematically investigated using an ab initio projected augmented plane-wave method. For structural stability studies, we have considered several possible structural modifications, and reproduced successfully the equilibrium structures for the known phases LiAlH 4 , NaAlH 4 , KAlH 4 , NaGaH 4 , and KGaH 4. Moreover, we predict the equilibrium structures of the other unknown members of this series. RbAlH 4 , CsAlH 4 , RbGaH 4 , and CsGaH 4 should crystallize with the KGaH 4-type structure, and LiGaH 4 should crystallize with the NaGaH 4-type structure. According to the density of states, all these compounds have nonmetallic character with a finite band gap of around 5 eV. Charge-density plot and electron localization function analyses show that the [BH 4 ] subunits almost look like a separate molecular species spread over the A matrix. An ionic type of interaction is present between the A and the [BH 4 ] units. Crystal orbital Hamilton population analyses reveal that the interaction between the B and H atoms is stronger than the other interactions present in these compounds.
Ceramics International, 2004
Using generalized-gradient-corrected full-potential density-functional calculations, we have stud... more Using generalized-gradient-corrected full-potential density-functional calculations, we have studied the electronic structure and magnetic properties of YBaMn 2 O 5 , Sr 4 Fe 4 O 11 , and Ca 3 Co 2 O 6. In these phases, the 3d transition metal ions have dual valence. We have studied the electronic structure using site-, angular momentum-, and orbital-projected density of states. The charge and orbital ordering are analyzed in terms of the calculated electron-density distribution, charge density, and electron localization function. The oxygen vacancy, cation radii, and crystal-field effects are found to play an important role for the various ordering phenomena in these compounds.
Variations in the spin, charge, and orbital ordering are analyzed in terms of the d-band filling ... more Variations in the spin, charge, and orbital ordering are analyzed in terms of the d-band filling for YBaT2O5 (T = Mn, Fe, Co). The calculations are performed with accurate density-functional-theory methods as implemented in the full-potential linearized-augmented plane-wave approach. We have carried out calculations for paramagnetic, ferromagnetic, and antiferromagnetic configurations. A ferrimagnetic ground state has been established for YBaMn2O5 whereas YBaFe2O5 and YBaCo2O5 have antiferromagnetic ground states in agreement with experimental findings. The electronic band characteristics are analyzed using total, site-, and orbital-projected density of states. Inclusion of spin-orbit coupling and Coulomb correlation effects are found to be important for YBaFe2O5 and YBaCo2O5 in order to reproduce the experimentally established semiconducting behavior. The different types of charge and orbital ordering in these compounds are visualized by making use of the energy-projected density m...
ACS Omega, 2019
Giant magnetoelectric coupling is a very rare phenomenon that has gained much attention in the pa... more Giant magnetoelectric coupling is a very rare phenomenon that has gained much attention in the past few decades due to fundamental interest as well as practical applications. Here, we have successfully achieved giant magnetoelectric coupling in PbTi 1−x V x O 3 (x = 0−1) using a series of generalized gradient-corrected GGA (generalized gradient approximation), including on-site Coulomb repulsion (U)-corrected spin-polarized calculations based on accurate density functional theory. Our total energy calculations show that PbTi 1−x V x O 3 stabilizes in C-type antiferromagnetic ground state for x > 0.123. With the substitution of V into PbTiO 3 , the tetragonal distortion is highly enhanced accompanied by a linear increase in polarization. In addition, our band structure analysis shows that for lower x values, the tendency to form two-dimensional magnetism of PbTi 1−x V x O 3 decreases. The orbital magnetic polarization was calculated with self-consistent field method by including orbital polarization correction in the calculation as well as from the computed X-ray magnetic dichroism spectra. A nonmagnetic metallic ground state is observed for the paraelectric phase for V concentration (x) = 1 competing with a volume change of 10% showing a large magnetovolume effect. Our orbital-projected density of states as well as orbital ordering analysis suggest that the orbital ordering plays a major role in the magnetic-to-nonmagnetic transition when going from ferroelectric to paraelectric phase. The calculated magnetic anisotropic energy shows that the direction [110] is the easy axis of magnetization for x = 1 composition. The partial polarization analysis shows that the Ti/V−O hybridization majorly contributes to the total electrical polarization. The present study adds a new series of compounds to the magnetoelectric family with rarely existing giant coupling between electric-and magnetic-order parameters. These results show that such kind of materials can be used for novel practical applications where one can change the magnetic properties drastically (magnetic to nonmagnetic, as shown here) with external electric field and vice versa.
Materials Research Express, 2019
Finding potential materials for solar cell applications is essential to reduce cost and enhance e... more Finding potential materials for solar cell applications is essential to reduce cost and enhance efficiency. We have employed Density Functional Theory (DFT) based calculations for novel nitrides of type A3N2 (A = Mg, Zn ans Sn) and Sn3N4 to find the ground state crystal and electronic structure. The structural parameters optimized by theoretical calculation are in good agreement with the experimental parameters. In order to obtain Sn3N2 and Sn3N4 experimentally, we are suggesting a new synthesis route from the calculated enthalpy of formation. The studied nitrides exhibit semiconductor behavior with direct band gaps. Band gap of 1.7 eV is obtained for Mg3N2 from GGA calculation. For Sn3N4 band gaps of 0.24 eV and 0.7 eV are obtained using GGA and LDA calculations, respectively, whereas GGA+U was used to obtain band gap in Zn3N2. The bonding behavior is analyzed in detail by using charge density and electron localization function plots. In addition, COHP (Crystal Orbital Hamiltonian Population) is utilized to retrieve bond strength values. Charge transfer from cation to anion decreases from Mg to Sn and correspondingly bond strength between the metal and Nitrogen atoms is also found to increase from Mg to Sn, which indicates increase in covalent nature of bonding from Mg to Sn. Zn3N2 is found to have the possibility for n-type as well as p-type doping because of the low effective mass of electrons and holes compared to other studied nitrides. Hence, Zn3N2 has suitable conductive properties to be used as a solar cell material.
Physical Review B, 2002
The oxygen-deficient (double) perovskite YBaMn 2 O 5 , containing cornerlinked MnO 5 square pyram... more The oxygen-deficient (double) perovskite YBaMn 2 O 5 , containing cornerlinked MnO 5 square pyramids, is found to exhibit ferrimagnetic ordering in its ground state. In the present work we report generalized-gradient-corrected, relativistic first-principles full-potential density-functional calculations performed on YBaMn 2 O 5 in the nonmagnetic, ferromagnetic and ferrimagnetic states. The charge, orbital and spin orderings are explained with site-, angular momentum-and orbital-projected density of states, charge-density plots, electronic structure and total energy studies. YBaMn 2 O 5 is found to stabilize in a G-type ferrimagnetic state in accordance with experimental results. The experimentally observed insulating behavior appears only when we include ferrimagnetic ordering in our calculation. We observed significant optical anisotropy in this material originating from the combined effect of ferrimagnetic ordering and crystal field splitting. In order to gain knowledge about the presence of different valence states for Mn in YBaMn 2 O 5 we have calculated K-edge x-ray absorption near-edge spectra for the Mn and O atoms. The presence of the different valence states for Mn is clearly established from the x-ray absorption near-edge spectra, hyperfine field parameters and the magnetic properties study. Among the experimentally proposed structures, the recently reported description based on P 4/nmm is found to represent the stable structure.
Physical Review B, 2003
A detailed high-pressure study on LiAlH 4 has been carried out using the ab initio projected augm... more A detailed high-pressure study on LiAlH 4 has been carried out using the ab initio projected augmented plane-wave method. Application of pressure transforms ␣to -LiAlH 4 (␣-NaAlH 4-type structure͒ at 2.6 GPa with a huge volume collapse of 17%. This abnormal behavior is associated with electronic transition from Al-s top states. At 33.8 GPa, a  to ␥ transition is predicted from ␣-NaAlH 4-type to KGaH 4-type structure. Up to 40 GPa LiAlH 4 remains nonmetallic. The high weight percent of hydrogen, around 22% smaller equilibrium volume, and drastically different bonding behavior than ␣-phase indicate that -LiAlH 4 is expected to be a potential hydrogen storage material.
Physical Review B, 2003
First-principle studies on the total energy, electronic structure, and bonding nature of RNiIn (R... more First-principle studies on the total energy, electronic structure, and bonding nature of RNiIn (RϭLa, Ce, and Nd͒, and their saturated hydrides (R 3 Ni 3 In 3 H 4 ϭRNiInH 1.333) are performed using a full-potential linear muffin-tin orbital approach. This series of phases crystallizes in a ZrNiAl-type structural framework. When hydrogen is introduced in the RNiIn matrix, anisotropic lattice expansion is observed along ͓001͔ and lattice contraction along ͓100͔. In order to establish the equilibrium structural parameters for these compounds we have performed force minimization as well as volume and c/a optimization. The optimized atomic positions, cell volume, and c/a ratio are in very good agreement with recent experimental findings. From the electronic structure and charge density, charge difference, and electron localization function analyses the microscopic origin of the anisotropic change in lattice parameters on hydrogenation of RNiIn has been identified. The hydrides concerned, with their theoretically calculated interatomic H-H distances of ϳ1.57 Å, violate the ''2-Å rule'' for H-H separation in metal hydrides. The shortest internuclear Ni-H separation is almost equal to the sum of the covalent radii. H is bonded to Ni in an H-Ni-H dumbbell-shaped linear array, with a character of NiH 2 subunits. Density of states, valence charge density, charge transfer plot, and electron localization function analyses clearly indicate significant ionic bonding between Ni and H and weak metallic bonding between H-H. The paired and localized electron distribution at the H site is polarized toward La and In which reduces the repulsive interaction between negatively charged H atoms. This could explain the unusually short H-H separation in these materials. The calculations show that all these materials have a metallic character.
Physical Review B, 2001
In order to understand the unexpected superconducting behavior of MgB 2 we have made electronic s... more In order to understand the unexpected superconducting behavior of MgB 2 we have made electronic structure calculations for MgB 2 and closely related systems. Our calculated Debye temperature from the elastic properties indicate that the average phonon frequency is very large in MgB 2 compared with other superconducting intermetallics and its exceptionally higher T c can be explained through a BCS mechanism only if phonon softening occurs or the phonon modes are highly anisotropic. We identified a doubly degenerate quasi-twodimensional key-energy band in the vicinity of E F along the ⌫-A direction of BZ ͑having equal amount of B p x and p y character͒ which plays an important role in deciding the superconducting behavior. Based on this result, we have searched for a similar electronic feature in isoelectronic compounds such as BeB 2 , CaB 2 , SrB 2 , LiBC, and MgB 2 C 2 and found that hole doped LiBC and MgB 2 C 2 are potential superconducting materials. We have found that E F in the closely related compound MgB 4 is lying in a pseudogap with a negligibly small density of states at E F , which is not favorable for superconductivity. There are contradictory experimental results regarding the anisotropy in the elastic properties of MgB 2 ranging from isotropic to moderately anisotropic to highly anisotropic. In order to settle this issue we have calculated the single-crystal elastic constants for MgB 2 by the accurate full-potential method and derived the directional-dependent linear compressibility, Young's modulus, shear modulus, and relevant elastic properties from these results. We have observed large anisotropy in the elastic properties consistent with recent high-pressure findings. Our calculated polarized optical dielectric tensor shows highly anisotropic behavior even though it possesses isotropic transport property. MgB 2 possesses a mixed bonding character and this has been verified from density of states, charge density, and crystal orbital Hamiltonian population analyses.
Solid State Communications, 2002
We have made a series of gradient-corrected relativistic full-potential density-functional calcul... more We have made a series of gradient-corrected relativistic full-potential density-functional calculation for Ca-substituted and hole-doped SrRuO3 in para, ferro, and A-, C-, and G-type antiferromagnetic states. Magnetic phase-diagram data for Sr1−xCaxRuO3 at 0 K are presented. Neutron diffraction measurement combined with total energy calculations show that spin-glass behavior with short-range antiferromagnetic interactions rules in CaRuO3. The substitution of Sr by Ca in SrRuO3 decreases the ferromagnetic interaction and enhances the G-type antiferromagnetic interaction; the G-AF state is found to stabilize around x = 0.75 consistent with experimental observations. Inclusion of spin-orbit coupling is found to be important in order to arrive at the correct magnetic ground state in ruthenates.
Physical Review Letters, 2002
Using gradient-corrected, all-electron, full-potential, density-functional calculations, includin... more Using gradient-corrected, all-electron, full-potential, density-functional calculations, including structural relaxations, it is found that the metal hydrides RT InH1.333 (R = La, Ce, Pr, or Nd; T = Ni, Pd, or Pt) possess unusually short H−H separations. The most extreme value (1.454Å) ever obtained for metal hydrides occurs for LaPtInH1.333. This finding violates the empirical rule for metal hydrides, which states that the minimum H−H separation is 2Å. Electronic structure, charge density, charge transfer, and electron localization function analyses on RT InH1.333 show dominant metallic bonding with a non-negligible ionic component between T and H, the H−H interaction being weakly metallic. The paired, localized, and bosonic nature of the electron distribution at the H site are polarized towards La and In which reduces the repulsive interaction between negatively charged H atoms. This could explain the unusually short H−H separation in these materials. Also, R−R interactions contribute to shielding of the repulsive interactions between the H atoms.
Physical Review B, 2006
Accurate ab initio density-functional calculations are performed to predict ground-state crystal ... more Accurate ab initio density-functional calculations are performed to predict ground-state crystal structures and to gain understanding of electronic structure and magnetic properties of CsCr 3 O 8 and ACr 3 O 8 ͑A =In,Tl,Cu,Ag,Au͒. CsCr 3 O 8 stabilizes in an orthorhombic ͑prototype; Pnma͒ structure in agreement with experimental findings whereas the remaining compounds stabilize in the monoclinic KCr 3 O 8-type ͑C2/m͒ structure. All compounds exhibit antiferromagnetic ordering in the ground state at 0 K. The electronic structures are analyzed with the help of density-of-states, charge-density, and electron-localization-function plots. All compounds ͑except InCr 3 O 8 ͒ are found to be semiconductors ͑insulators at 0 K͒ with very small band gaps, and Cr atoms in different environments consistently take different valence states.
Physical Review B, 2004
Using generalized-gradient-corrected full-potential density-functional calculations we have studi... more Using generalized-gradient-corrected full-potential density-functional calculations we have studied the magneto-optical properties of double perovskites A 2 BBЈO 6 , A = Ca, Sr, and Ba; B = Fe; and BЈ = Mo, W, and Re. Sr 2 FeWO 6 has the maximum polar Kerr rotation of 3.87°and specific Faraday rotation of 4.5 ϫ 10 5 deg cm −1. All other compounds have Kerr rotation more than 1°, except Ca 2 FeMoO 6 which has maximum Kerr rotation of 0.5°. Our electronic structure studies show that all these compounds are half-metallic in the ferromagnetic configuration. The large Kerr rotation is found to be due to the combined effects of relatively large exchange splitting from Fe, large spin-orbit coupling due to 4d or 5d elements, large offdiagonal conductivity, and half-metallicity.
Physical Review B, 2008
The structural, electronic, and magnetic properties of the oxygen-vacancy-ordered mixed-valence S... more The structural, electronic, and magnetic properties of the oxygen-vacancy-ordered mixed-valence Sr 4 Fe 4 O 11 phase have been investigated using spin-polarized electronic-structure total-energy calculations. The optimized structural parameters obtained from accurate total-energy calculations are found to be in very good agreement with low-temperature neutron-diffraction findings. Among the different spin configurations considered for Sr 4 Fe 4 O 11 , the G-type antiferromagnetic configuration is found to represent the magnetic ground state. The calculations show finite magnetic moments at both Fe sites, and this is against the conclusions arrived at from Mössbauer spectroscopy and low-temperature powder neutron-diffraction measurements, but consistent with the results from magnetization measurements. The present study clearly show that one of the magnetic sublattices is frustrated and hence Sr 4 Fe 4 O 11 can be considered as a phase-separated system with one phase in the G-type antiferromagnetic state and the other in a spin-glass-like state. Our theoretical results show unambiguously that Fe atoms with the square pyramidal environment have a lower oxidation state than that in the octahedral coordination. However, the presence of a covalent interaction between Fe and the neighboring oxygen atoms makes the actual oxidation state considerably smaller than the formal oxidation state of 3+ and 4+ for square-pyramidal and octahedral coordination, respectively. Sr 4 Fe 4 O 11 is found to be semiconducting in the antiferromagnetic ground state.
Physical Review B, 2006
Ground-state crystal structures are predicted for Cr 3 O 8 and LiCr 3 O 8 from accurate first-pri... more Ground-state crystal structures are predicted for Cr 3 O 8 and LiCr 3 O 8 from accurate first-principles densityfunctional calculations by considering several ͑25 for the former and 12 for the latter phase͒ related structure types. Both phases stabilize in monoclinic ͑C2/m͒ structures with slight variations in the atomic arrangement of the chromium-oxygen framework. Structural optimization is also performed for Cr 8 O 21 , the obtained structural parameters being in good agreement with the experimental values. A possible synthesis route for Cr 3 O 8 is suggested from total energy studies. Electronic structure studies indicate insulating behavior with very small band gaps for Cr 8 O 21 and LiCr 3 O 8 whereas Cr 3 O 8 has pseudogaplike feature. Magnetic property and partial density of states analyses indicate two different valence states for Cr in all studied phases ͑except CrO 3 ͒. Bonding characteristics and effects of Li intercalation in Cr 3 O 8 are also analyzed with the help of charge density and charge transfer plots.
Physical Review B, 2008
From analyses of structural information for oxides with Fe in different oxidation states and comp... more From analyses of structural information for oxides with Fe in different oxidation states and computationally estimated Mössbauer parameters ͑hyperfine field, isomer shift, and quadrupole splitting͒ based on densityfunctional theory, we show that the charges residing on the different constituents cannot be directly derived either from structural or Mössbauer measurements. We have analyzed charge density, charge transfer, electron localization function, crystal orbital Hamilton population, and partial density of states to explain the bonding characteristics. Born-effective charge tensor is used to quantify the charges present at the atomic sites in Sr 4 Fe 4 O 11. We show that the effects of covalence are important in explaining the electronic structure, magnetism, and chemical bonding in oxygen-vacancy-ordered systems such as Sr 4 Fe 4 O 11 and on ignoring covalence, one can be misled in oxidation-state assignments.
Journal of Solid State Chemistry, 2003
The development in theoretical condensed-matter science based on density-functional theory (DFT) ... more The development in theoretical condensed-matter science based on density-functional theory (DFT) has reached a level where it is possible, from ''parameter-free'' quantum mechanical calculations to obtain total energies, forces, vibrational frequencies, magnetic moments, mechanical and optical properties and so forth. The calculation of such properties are important in the analyses of experimental data and they can be predicted with a precision that is sufficient for comparison with experiments. It is almost impossible to do justice to all developments achieved by DFT because of its rapid growth. Hence, it has here been focused on a few advances, primarily from our laboratory. Unusual bonding behaviors in complex materials are conveniently explored using the combination of charge density, charge transfer, and electron-localization function along with crystal-orbital Hamilton-population analyses. It is indicated that the elastic properties of materials can reliably be predicted from DFT calculations if one takes into account the structural relaxations along with gradient corrections in the calculations. Experimental techniques have their limitations in studies of the structural stability and pressure-induced structural transitions in hydride materials whereas the present theoretical approach can be applied to reliably predict properties under extreme pressures. From the spin-polarized, relativistic full-potential calculations one can study novel materials such as ruthenates, quasi-one-dimensional oxides, and spin-, charge-, and orbitalordering in magnetic perovskite-like oxides. The importance of orbital-polarization correction to the DFT to predict the magnetic anisotropy in transition-metal compounds and magnetic moments in lanthanides and actinides are emphasized. Apart from the fullpotential treatment, proper magnetic ordering as well as structural distortions have to be taken into account to predict correctly the insulating behavior of transition-metal oxides. The computational variants LDA and GGA fail to predict insulating behavior of Mott insulators whereas electronic structures can be described correctly when correlation effects are taken into account through LDA þ U or similar approaches to explain their electronic structures correctly. Excited-state properties such as linear optical properties, magneto-optical properties, XANES, XPS, UPS, BIS, and Raman spectra can be obtained from accurate DFT calculations.
Journal of Crystal Growth, 2004
The role of structural distortion, magnetic ordering, and Coulomb-correlation effect on the elect... more The role of structural distortion, magnetic ordering, and Coulomb-correlation effect on the electronic structure of perovskite-like oxides is analyzed. The density-functional theory (DFT) is originally devised to describe the groundstate properties of materials. However, during recent years it has been seen that the DFT can also be used to study excited state properties succesfully. We have recently calculated the electronic structure and linear optical properties for the series LaXO 3 (X ¼ Sc-Cu) and found that the gradient-corrected DFT describes correctly the insulating behavior of the ionic insulator LaScO 3 and the charge-transfer insulators LaCrO 3 ; LaFeO 3 ; and LaMnO 3 although the band gaps are systematically underestimated. For example, the good agreement between experimental and theoretical reflectivity spectra for LaCrO 3 clearly demonstrates that accurate full-potential DFT calculations not only describe the occupied and unoccupied states of the bands well, but also reproduce their characters. We have also calculated XPS, XANES, and magneto-optical spectra for perovskite-like oxides. For Mott-Hubbard insulators such as LaTiO 3 and LaVO 3 the DFT failed to predict insulating behavior and here the LDA+U method is applied to describe the electronic structure correctly.
Journal of Applied Physics, 2012
First-principles density functional calculations have been performed on Li-doped ZnO using allele... more First-principles density functional calculations have been performed on Li-doped ZnO using allelectron projector augmented plane wave method. Li was considered at six different interstitial sites (Li i), including anti-bonding and bond-center sites and also in substitutional sites such as at Zn-site (Li zn) and at oxygen site (Li o) in the ZnO matrix. Stability of Li Zn over Li i is shown to depend on synthetic condition, viz., Li Zn is found to be more stable than Li i under O-rich conditions. Hybrid density functional calculations performed on Li Zn indicate that it is a deep acceptor with (0/-) transition taking place at 0.74 eV above valence band maximum. The local vibrational frequencies for Li-dopants are calculated and compared with reported values. In addition, we considered the formation of Li-pair complexes and their role on electronic properties of ZnO. Present study suggests that at extreme oxygen-rich synthesis condition, a pair of acceptor type Li Zn-complex is found to be stable over the compensating Li i þ Li Zn pair. The stability of complexes formed between Li impurities and various intrinsic defects is also investigated and their role on electronic properties of ZnO has been analyzed. We have shown that a complex between Li Zn and oxygen vacancy has less formation energy and donor-type character and could compensate the holes generated by Li-doping in ZnO. V
Europhysics Letters (EPL), 2005
To design new hydrogen storage materials a main prerequisite is the knowledge about the site pref... more To design new hydrogen storage materials a main prerequisite is the knowledge about the site preference of hydrogen in the corresponding nonhydride phases. From the systematic investigation of ZrNiAl-type phases we found that the electron-localization function (ELF) can correctly predict the site preference of hydrogen in metallic phases. From the site preference trends of H in ZrNiAl-type phases we have shown that hydrogen prefers to occupy interstitial sites where ELF indicates a maximum value. Based on this observation we generalized an empirical site-preference rule which states that hydrogen prefers to occupy the interstitial sites where electrons have relatively more nonbonding localized nature than other possible sites in metals, alloys and intermetallic frameworks.
Crystal Growth & Design, 2004
The crystal and electronic structures of the entire series of alkali aluminum and alkali gallium ... more The crystal and electronic structures of the entire series of alkali aluminum and alkali gallium tetrahydrides (ABH 4 ; A) Li, Na, K, Rb, or Cs; B) Al or Ga) are systematically investigated using an ab initio projected augmented plane-wave method. For structural stability studies, we have considered several possible structural modifications, and reproduced successfully the equilibrium structures for the known phases LiAlH 4 , NaAlH 4 , KAlH 4 , NaGaH 4 , and KGaH 4. Moreover, we predict the equilibrium structures of the other unknown members of this series. RbAlH 4 , CsAlH 4 , RbGaH 4 , and CsGaH 4 should crystallize with the KGaH 4-type structure, and LiGaH 4 should crystallize with the NaGaH 4-type structure. According to the density of states, all these compounds have nonmetallic character with a finite band gap of around 5 eV. Charge-density plot and electron localization function analyses show that the [BH 4 ] subunits almost look like a separate molecular species spread over the A matrix. An ionic type of interaction is present between the A and the [BH 4 ] units. Crystal orbital Hamilton population analyses reveal that the interaction between the B and H atoms is stronger than the other interactions present in these compounds.
Ceramics International, 2004
Using generalized-gradient-corrected full-potential density-functional calculations, we have stud... more Using generalized-gradient-corrected full-potential density-functional calculations, we have studied the electronic structure and magnetic properties of YBaMn 2 O 5 , Sr 4 Fe 4 O 11 , and Ca 3 Co 2 O 6. In these phases, the 3d transition metal ions have dual valence. We have studied the electronic structure using site-, angular momentum-, and orbital-projected density of states. The charge and orbital ordering are analyzed in terms of the calculated electron-density distribution, charge density, and electron localization function. The oxygen vacancy, cation radii, and crystal-field effects are found to play an important role for the various ordering phenomena in these compounds.