V. Vyas

Ab-initiostudy of structural and electronic properties of AlAs

The structural properties, i.e. equilibrium lattice constant, transition pressure, bulk modulus and its pressure derivatives, together with electronic properties, i.e. energy bands, Compton profile and autocorrelation function, of AlAs are presented in this work. The linear combination of atomic orbitals (LCAO) method of the CRYSTAL code was applied considering the Perdew–Burke–Ernzerhof correlation energy functional and Beckes ansatz for the exchange. The total energy of AlAs as a function of primitive cell volume has also been calculated for the zincblende (B3), nickel arsenide (B8), sodium chloride (B1) and cesium chloride (B2) phases. Structural parameters of the B3, B8, B1 and B2 phases are determined. The calculated structural parameters are found to be in good agreement with the results of previous investigations. The spherically averaged theoretical values of Compton profile are in good agreement with an earlier measurement. The LCAO calculation shows an indirect band gap of 1.85 eV, in reasonable agreement with earlier data. On the basis of the equal-valence-electron-density Compton profile, it is found that AlAs is more ionic compared to AlSb.

Electron momentum density and phase transition in SrO

In this article, we present electron momentum density distribution and phase transition in SrO. The experimental values of momentum density have been measured using 5Ci 241Am Compton spectrometer and analyzed using theoretical data obtained from the ab-initio linear combination of atomic orbitals method. The first-principles calculations of the total energy of SrO as a function of cell volume have also been carried out for the cubic rocksalt (B1) and cesium chloride (B2) phases. Several structural parameters, i.e. equilibrium lattice constant, transition pressure, bulk modulus, etc. of B1 and B2 phases have been calculated and compared with the previous investigations. We conclude that the stable phase of SrO is B1 and the phase transition from B1 to B2 occurs at 35.8 GPa.

Ab-initio study of the electronic and elastic properties of beryllium chalcogenides BeX (X=S, Se and Te)

Ab-initio methods have been employed to investigate the electronic and elastic properties of beryllium chalcogenides (namely BeS, BeSe and BeTe). The electron momentum density, autocorrelation function and energy band gap have been computed using the linear combination of atomic orbitals method. Using the full potential linearized augmented plane-wave and projector-augmented wave methods, the energy bands and density of states (DOS) along with elastic properties are also calculated. The electronic band structure, total and partial DOS and elastic moduli obtained from the present calculations are found to be in good agreement with available earlier data. The calculated valence band width, equal valence electron density curve and bulk modulus confirm the trend of ionicity BeS>BeSe>BeTe.

A study of electron momentum density distribution in Al2O3 ceramic

A study of electron momentum density distribution in α-Al2O3 ceramic using Compton spectroscopy is presented in this work. Measurements have been carried out using 59.54 keV gamma-rays emanating from an Am241 source. Calculations have been performed on the basis of the ab-initio linear combination of atomic orbitals (LCAO) method embodied in the CRYSTAL code. The correlation scheme proposed by Perdew–Burke–Ernzerhof was adopted. The exchange was treated following the Becke scheme. The Hartree–Fock and hybrid schemes were also applied to the compound. All the schemes yielded results that are in good agreement with the measurements. The agreement with experiment is, however, better with the hybrid B3LYP (Lee–Yang–Parr) scheme. Ionic model calculations for a number of configurations of (Al+x)2(O−2x/3)3 (2.75≤x≤3 in steps of 0.125) were also performed utilizing free atom profiles. The ionic model suggests transfer of 2.875 electrons from the valence sp state of Al to the p state of O.

Electronic structure of nano-sized ZnO: A Compton profile study

The electronic structure of nano-sized ZnO using Compton scattering technique is reported. The nano-crystalline ZnO of size 10.5 nm is synthesized using the chemical route and characterized by XRD, SEM, TEM probes. Using 59.54 keV gamma-rays, the Compton profile measurements are performed on nano-sized as well as bulk ZnO. The present study reveals the narrower electron momentum density in nano-sized ZnO as compared to bulk sample. The ionic model based free atom Compton profiles for a number of configurations of Zn+xO−x (0.0 ≤ x ≤ 2.0) are also performed to estimate the charge transfer on compound formation. The present study suggests different amount of charge transfer from Zn to O atom in the nano-sized and bulk ZnO.

First-principles study of B1 to B2 phase transition in PbS

The high pressure structural phase transition in PbS has been studied by means of first-principles total energy calculations which are based on linear combination of atomic orbitals (LCAO) method within local density approximation (LDA). In the present study, the exchange scheme of Becke and correlation functional of von-Barth-Hedin (VBH) are employed. It is observed that more stable phase for PbS is NaCl type (B1) and PbS transforms to the CsCl type (B2) structure under high pressure (22.8 GPa). The calculated value of transition pressure (Pt) from B1 to B2 structure is found in good agreement with the earlier experimental and theoretical investigations.

Study of phase transition and cohesive energy in MgO

In this paper, we present first-principles study of phase transition and cohesive energy of MgO. The calculations are performed on the basis of density functional theory (DFT) based on linear combination of atomic orbitals (LCAO) method. In the present calculations, the generalized gradient approximation (GGA) proposed by Perdew-Burke- Ernzerhof (PBE) and Beckes scheme are considered to treat the correlation and exchange effects respectively. The study reveals that the stable phase of MgO is B1. The study also suggests B1 to B2 phase transition at 260.75 GPa and 10.98 eV cohesive energy of MgO in the stable phase. The calculated data are found in good agreement with the previous theoretical and experimental investigations.

Electron momentum density, band structure, and structural properties of SrS

The electron momentum density, the electronic band structure, and the structural properties of SrS are presented in this paper. The isotropic Compton profile, anisotropies in the directional Compton profiles, the electronic band structure and density of states are calculated using the ab initio periodic linear combination of atomic orbitals method with the CRYSTAL06 code. Structural parameters of SrS—lattice constants and bulk moduli in the B1 and B2 phases—are computed together with the transition pressure. The computed parameters are well in agreement with earlier investigations. To compare the calculated isotropic Compton profile, measurement on polycrystalline SrS is performed using 5Ci-241Am Compton spectrometer. Additionally, charge transfer is studied by means of the Compton profiles computed from the ionic model. The nature of bonding in the isovalent SrS and SrO compounds is compared on the basis of equal-valenceelectron-density profiles and the bonding in SrS is found to be more covalent than in SrO.

Compton Profile Study of Intermetallic Ti3Al

The Compton scattering measurement on intermetallic alloy Ti3Al is reported in this work. The measurement is made using 59.54 keV gamma‐rays from Am241 source. Theoretical calculation of Compton profile is also performed employing CRYSTAL code within the framework of density functional theory to compare with the measurement. The theoretical profile of the alloy is also synthesized following the superposition model taking the published Compton profiles of elemental solids from the APW method. The experimental study of charge transfer in the alloys has also been done by performing the experimental Compton profile measurements on Ti and Al following the superposition model and charge transfer from Al to Ti is clearly seen on the alloy formation.

Electronic Properties of Beryllium‐Chalcogenides BeX (S, Se and Te)

A first‐principles method has been employed to investigate the electronic properties of Be‐chalcogenides namely BeS, BeSe and BeTe. The electron momentum density, autocorrelation function, energy band gap and density of states have been computed using linear combination of atomic orbitals (LCAO) method. The calculated electronic energy band gap and density of states are found to be in good agreement with available earlier data. The bonding in the BeX compounds also been compared on the basis of equal‐valence‐electron‐density profiles and the trend of ionicity is found as BeS>BeSe>BeTe.