U. P. Verma

Pressure effects on energy gaps and phase transitions in ZnAl2Se4

Abstract The full potential linear augmented wave method has been used for an ab-initio study of structural, electronic and high pressure phase transformation features of ZnAl2Se4. The exchange and correlation potential (XC) was calculated using the Perdew–Burke–Ernzerhof generalized gradient approximation (PBE-GGA) and the modified GGA due to Engel–Vosko (EV-GGA). The geometry optimization was performed by calculating the total energy as a function of the three variables u, c/a and V. Using the sets of data obtained we determined the equilibrium volume, V0, the bulk modulus, B0, and the pressure derivative of the bulk modulus, B0′. The results show that ZnAl2Se4 is a direct wide band gap semiconductor at ambient conditions (P = 0, T = 0). With increasing hydrostatic pressure of the unit cell (i.e., with decreasing size) both the direct (Γ—Γ) and the indirect (Γ—H) energy gaps first increase and at a pressure of 9.2 GPa both gaps become equal and start decreasing as the pressure increases further but with a change in the nature of the energy band gap from direct to indirect. With increasing pressure a transition from tetragonal to spinel phase has been observed at a pressure of 3 GPa.

Ab initio studies of structural, electronic, magnetic and mechanical properties of alkali earth metal silicides

Alkali earth metal silicides MSi (M = Mg, Ca, Sr, Ba) are multi-phase compound and exist simultaneously in CrB, CsCl, NaCl or rock salt (RS) and zinc blende (zb) structures. In the CrB and CsCl phases, their behavior is metallic in the non-magnetic (NM) as well as the ferromagnetic (FM) structure. The total spin magnetic moment of these compounds in the zb phase is more than that in the corresponding RS phase; therefore, detailed studies for the zb phase are presented in this paper. This study includes structural, electronic and mechanical properties by using the full potential linear augmented plain wave scheme with local orbitals. Ferromagnetic CaSi, SrSi and BaSi show true half-metallic character. For a better understanding of the half-metallicity in the above-mentioned sp-type compounds, their band structures have been calculated and densities of state plots have been produced. The FM structures are more stable and harder than the corresponding NM structures. The magnetic moment corresponding to equilibrium lattice constants is calculated as 2 µB for FM CaSi, SrSi and BaSi, which are in accordance with the earlier work on the sp-type compounds CaC, SrC and BaC. The FM character changes to the paramagnetic character as the lattice parameter decreases. The general trend is that the values of the elastic constants C11, C12 and C44 increase with increasing hydrostatic pressure.

Should muffin tin radius vary in different structures of a material?: A case study

Quantum mechanical calculations based on density functional theory and a generalized gradient approximation (GGA) have been used to study the structural properties of YbN. Its predicted unit cell lattice parameter in NaCl (B1) structure is 4.7810A and in CsCl (B2) structure it is 2.8685A. In the determination of lattice parameter the muffin tin radius (RMT) of constituent atoms play important role. In both the structures the muffin tin radius for Yb and N converges to 2.3 and 1.4 a.u., respectively.

Ab initio study of the properties of CuAlSe2: A chalcopyrite compound

Abstract The ground-state structural, electronic, optical and thermal properties of CuAlSe2 compound in the body centered tetragonal phase have been studied by employing the full potential linearized augmented plane wave plus local orbitals method based on the density functional theory. For the exchange-correlation effects we have adopted the local density approximation (LDA), generalized gradient approximation (GGA) due to Perdew, Burke and Ernzerhof (PBE) and Engel and Vosko (EV). At the equilibrium unit cell volume, ration of the lattice constants, c/a, has been obtained as 1.99 and the unit cell constant a0 = 5.51Å which are well comparable with the experimental results. The energy band gap obtained using EVGGA (1.92 eV) is in good agreement in comparison to that using LDA (1.09 eV), GGA (PBE) (1.26 eV) and other earlier available data. Upper portion of the valance band is mainly composed of Cu-d and Se-p states and they are highly hybridized. The optical properties such as dielectric function, refraction, reflection, conductivity, absorption and loss of energy of CuAlSe2 are presented. The quasi Harmonic Debye model is applied to study the thermal and vibrational effects for the stable BCT CuAlSe2. Thermal parameters such as Gruneisen parameter, Debye temperature, specific heat etc have been determined as a function of pressure and temperature.

Physical properties of molybdenum monoboride: Ab-initio study

Abstract The Ab initio investigations on structural, electronic, optical and thermal properties of MoB have been reported using full potential linearised-augmented plane wave method within the framework of density functional theory. The exchange and correlation potentials were calculated using the Perdew-Burke-Ernzerhof-Sol generalised gradient approximation. The calculated equilibrium lattice constants and cell volume are in excellent agreement with the experimental results as compared to the available theoretical data. Electronic band structure shows that MoB is metallic in nature. From the partial densities of states of MoB it has been found that major contribution on the Fermi level is due to Mo-4d states. Among the reported optical parameters the large value of reflectivity at low energy shows that MoB can be used as a coating material in IR region. Maximum absorption in extreme UV region shows that it can be used in production of electricity through solar power in space vehicles. Various thermal properties have been calculated in a wide temperature range at high pressures. Change in thermal expansion coefficient with respect to temperature shows that anharmonic effect in MoB is very weak at high temperature. The optical and thermal properties of MoB are presented for the first time in this work.

A first-principle approach to study the structural and elastic properties of alkali hydrides

The comparative study of structural stability and elastic constants of alkali hydrides (LiH, NaH, KH) and their extra added hydrogen compounds (LiH+2H, NaH+2H and KH+2H) in NaCl structure has been presented. Calculations have been performed using first-principles approach and employing full potential linearized augmented plane wave (FP-LAPW) method. Generalized gradient approximation (GGA) has been used for solving exchange correlation functional. Our obtained elastic constants of LiH, NaH and KH are in good agreement with reported experimental and other theoretical data.

Hydrogen storage in LiH: A first principle study

First principles calculations have been performed on the Lithium hydride (LiH) using the full potential linearized augmented plane wave (FP-LAPW) method within the framework of density functional theory. We have extended our calculations for LiH+2H and LiH+6H in NaCl structure. The structural stability of three compounds have been studied. It is found that LiH with 6 added Hydrogen atoms is most stable. The obtained results for LiH are in good agreement with reported experimental data. Electronic structures of three compounds are also studied. Out of three the energy band gap in LiH is ∼3.0 eV and LiH+2H and LiH+6H are metallic.

Bandedge emission in sprayed ZnO thin films

AbstractZnO thin films were grown by spraying aqueous solutions of zinc acetate on quartz and silicon substrates heated to 500°C. X-ray diffraction (XRD) studies show that the ZnO films crystallize in the hexagonal phase and are highly oriented along the [002] direction. SEM micrographs show that the surface of the films is uniformly covered with worm shaped particles. EDAX results together with XRD studies confirm that the synthesized films are single phase ZnO with good stoichiometry. The sharp absorption occurring near the band edge indicates high quality growth of the material. Room temperature photoluminescence (PL) studies of the sprayed films show a strong near band edge (NBE) emission. PL spectra obtained at low temperature (20 K) exhibit better resolved peaks of the NBE emission caused by the radiative recombination of free and bound excitons. The dominant free exciton emission observed in the PL spectra confirms the high quality of the sprayed ZnO layers.

Study of Structural and Electronic Behavior of BeH2 as Hydrogen Storage Compound: An Ab Initio Approach

The quantum mechanical calculations based on density functional theory (DFT) have been performed to study ground state structural and electronic properties of BeH2 and along with doping of two (BeH2

Ab-initio study of Al1–xGaxSb compound

Abstract The ground-state structural, electronic and optical properties of the Al1–xGaxSb compound in the zinc-blende phase for x = 0.125, 0.25 and 0.50 have been stud ied employing the full potential linearized augmented plane wave plus local orbitals method based on the density functional theory. For the exchange-correlation effects we have adopted the GGA approach. In order to model Al1–xGaxSb zincblende alloys, we employ a 16-atoms supercell of the type 2 × 2 × 2. The lattice structures of Al0.875Ga0.125Sb, Al0.75Ga0.25Sb and Al0.50Ga0.50Sb are obtained by replac ing one, two and four Al atoms, respectively, with an equal number of Ga atoms in the crystal lattice of AlSb. The variation of energy band gap of Al1–xGaxSb versus the value of ‘x’ reveals that the band gap bowing para m eter has strong composition dependence for small concentrations of Ga. The DOS plots and the various calculated optical properties are in accordance with the calculated electronic properties. The characteristic properties of Al1–xGaxSb are dominated by the Sb 5p electrons. Linear optical properties such as the dynamic dielectric function, static refractive index, reflectivity and energy loss function for an energy range of 0 to 13.50 eV have been studied. The static dielectric constant increases with the increase in Ga concentration in Al1–xGaxSb. The bulk plasma frequency ωp for this material corresponds to 13.19 eV.