F. El Haj Hassan

Full potential linearized augmented plane wave calculations of structural and electronic properties of BN, BP, BAs and BSb

A theoretical study of structural and electronic properties of boron compounds BN, BP, BAs and BSb is presented, using the full potential linearized augmented plane wave method. In this approach, the generalized gradient approximation was used for the exchange-correlation potential. Ground state properties such as lattice parameter, bulk modulus and its pressure derivative are calculated as well as structural transition pressure. The band structure is obtained for both zincblende and rocksalt structures. We also give the valence charge density at equilibrium lattice constant and at transition pressure. We show from the latter quantity the inverse role between cation and anion for BP, BAs and BSb. Results are discussed and compared with experimental and other theoretical data with reasonable agreement.

High pressure study of structural and electronic properties of calcium chalcogenides

The structural and electronic properties of calcium chalcogenides CaX (X = S,Se,Te) under high pressure have been investigated using the full potential linearized augmented plane wave method within density functional theory. We used both the local density approximation and the generalized gradient approximation (GGA) that is based on exchange?correlation energy optimization for calculating the total energy. Moreover, the Engel?Vosko GGA formalism is applied so as to optimize the corresponding potential for band structure calculations. The equilibrium lattice constant for CaX compounds agrees well with the experimental results. The pressures at which these compounds undergo a structural phase transition from NaCl-type to CsCl-type were calculated. A numerical first-principles calculation of the elastic constants was used to calculate C11, C12 and C44. The energy band gaps at ambient conditions in the NaCl-type structure and the volume dependence of band gaps in the CsCl-type structure up to the band overlap metallization were investigated. Besides this, the nature of the chemical bond in these compounds was analysed in terms of electronic charge density.

Structural properties of copper halides

Abstract Full potential linearized augmented plane wave method (FP-LAPW) is applied to study the elastic and the bonding properties of CuCl, CuBr, and CuI. A numerical first principles calculation of the elastic constants is used to calculate C 11 , C 12 , and C 44 for these compounds. The equilibrium lattice constant and the modulus are also given. Our FP-LAPW calculations were performed to evaluate the total energy as a function of strain, after which the data were fitted to a polynomial function of strain to determine the modulus. The experimental results of elastic constants are available only for CuCl. For CuBr and CuI, the results are compared with the available theoretical works with good agreement. In addition, we discuss the bonding character in terms of charge density, which show the strong localisation of charge around the anion side. The calculated ionicity parameter agrees with the expected estimation from the charge density behavior and compared well with the Phillips’ ionicity scale.

Structural properties of boron compounds at high pressure

The present work employs the full potential linearized augmented plane wave (FP-LAPW) method to calculate the elastic and the bonding properties of the boron compounds BN, BP, BAs and BSb at high pressures. A numerical first principles calculation of the elastic constants is used to obtain C11, C12 and C44 for these compounds. The equilibrium lattice constant and the bulk modulus are also calculated. Our FP-LAPW calculations were performed to evaluate the total energy as a function of strain, after which the data were fitted to a polynomial function of strain to determine the elastic modulus. The experimental elastic constants are only available for BN and BP. Hence for BAs, the results are only compared with the available theoretical works. To our knowledge the elastic constants of BSb have not yet been measured or calculated, hence our results serve as a prediction for future study. In addition, we discuss the bonding parameter in terms of charge density at equilibrium and at transition volumes, which suggests that the bonding of BP, BAs and BSb are less ionic than in other zinc-blende compounds.

Structural and electronic properties of the wide-gap Zn1−xMgxS, Zn1−xMgxSe and Zn1−xMgxTe ternary alloys

Zn1−x Mg x S, Zn1−x Mg x Se and Zn1−x Mg x Te ternary wide-gap semiconductor alloys were investigated using the full potential–linearized augmented plane wave (FP-LAPW) method. We have studied the effect of composition on structural properties such as lattice constants, bulk modulus and bond ionicity. The bandgap and the microscopic origins of compositional disorder have also been explained in detail. In addition, from the obtained band structures, the electron (hole) conduction and valence effective masses are deduced. These parameters were found to depend non-linearly on alloy composition x ,e xcept the lattice parameter for Zn1−x Mg x S, which follows Vegard’s law. The calculated band structures for all three alloys show a direct bandgap in the whole range of x composition. We have paid special attention to the disorder parameter (gap bowing). Using the approach of Zunger and co-workers, we have concluded that the total bandgap energy bowing was mainly caused by the charge exchange effect for the alloys of interest.

Theoretical study of structural and electronic properties of CaFI

The full potential linearized augmented plane wave (FP-LAPW) method within density functional theory is applied to study, for the first time, the structural and electronic properties of CaFI and to compare them with CaFCl and CaFBr, all compounds belonging to the tetragonal PbFCl structure group with space group P4/nmm. We used the generalized gradient approximation (GGA) based on exchange?correlation energy optimization to calculate the total energy and also the Engel?Vosko GGA formalism, which optimizes the corresponding potential for band structure calculations. Ground state properties such as the lattice parameters, c/a ratio, bulk modulus, pressure derivative of the bulk modulus and cohesive energy are calculated as well as the optimized internal parameters, by relaxing the atomic position in the force directions. The variations of the calculated interatomic distances and angles between different atomic bonds are discussed. CaFCl was found to have a direct band gap at ? whereas CaFBr and BaFI have indirect band gaps. From these computed bands, all three materials are found to be insulators having band gaps of 6.28, 5.46 and 4.50?eV, respectively. We also calculated the valence charge density and the total density of states at equilibrium volume for each compound. The results are in reasonable agreement with the available experimental data.

Structural, electronic, thermodynamic and optical properties of alkaline earth oxides MgO, SrO and their alloys

The structural, electronic, optical and thermodynamic properties of Mg1 xSrxO ternary alloys in NaCl structures at various Sr concentrations are presented. The calculations were performed using the full potential linearized augmented plane wave (FP-LAPW) method within the density functional theory (DFT) in the local density approximation (LDA) and two developed refinements, namely the generalized gradient approximation (GGA) of Perdew et al (1996 Phys. Rev. Lett. 77 3865) for the structural properties and Engel and Vosko (1993 Phys. Rev. B 47 13164) for the band structure calculations. Deviation of the lattice constants from Vegard’s law and the bulk modulus from the linear concentration dependence (LCD) were observed for the alloys. The microscopic origins of the gap bowing were explained by using the approach of Bernard and Zunger (1986 Phys. Rev. Lett. 34 5982). The refractive index and optical dielectric constant for the alloys of interest were calculated by using different models. In addition, the thermodynamic stability of the alloys was investigated by calculating the critical temperatures of alloys.

FIRST-PRINCIPLES INVESTIGATION OF SnO2 AT HIGH PRESSURE

The ground state properties and the structural phase transformation of tin dioxide (SnO2) have been investigated using first principle full potential-linearized augmented plane wave (FP-LAPW) method within density functional theory (DFT). We used local density approximation (LDA) and the generalized gradient approximation (GGA), which are based on exchange-correlation energy optimization, to optimize the internal parameters by relaxing the atomic positions in the force directions and to calculate the total energy. For band structure calculations, we utilized both the Engel-Voskos generalized gradient approximation (EVGGA), which optimizes the exchange-correlation potential, and also GGA. From the obtained band structures, the electron (hole) valance and conduction effective masses are deduced. For compressed volumes SnO2 is shown to undergo two structural phase transitions with increasing pressure from the rutile- to the CaCl2-type phase at 12.4 GPa and to a cubic phase, space group at 22.1 GPa. The calculated total energy allowed us to investigate several structural properties, in particular, the equilibrium lattice constants, bulk modulus, cohesive energy, interatomic distances and the angles between different atomic bonds. In addition, we discuss the bonding parameter in term of charge density, which show the localization of charge around the anion side.

Structural, electronic and thermodynamic properties of magnesium chalcogenide ternary alloys

The full potential–linearized augmented plane wave (FP-LAPW) method within the density functional theory (DFT) was applied to study the structural, electronic and thermodynamic properties of MgSxSe1−x, MgSxTe1−x and MgSexTe1−x ternary alloys. The calculated lattice parameters at different compositions of MgSxSe1−x and MgSexTe1−x alloys were found to vary almost linearly, while a significant deviation of the lattice parameter from Vegards law for MgSxTe1−x alloy was observed. This is mainly due to the large mismatch of the lattice parameters of the binary compounds MgS and MgTe. A large deviation of the bulk modulus from linear concentration dependence (LCD) was observed for all three alloys. The calculated optical bowing was found to be mainly caused by the structural relaxation. Moreover, a significant charge exchange contribution was observed in the case of MgSxTe1−x alloy. The calculated phase diagram shows a broad miscibility gap for these alloys with a high critical temperature.

FP-LAPW investigations of SrS1−xSex, SrS1−xTex and SrSe1−xTex ternary alloys

The ab initio full potential linearized augmented plane wave (FP-LAPW) method within density functional theory (DFT) was applied to study the effect of composition on the structural, electronic, optical and thermodynamic properties of SrS1−xSex, SrS1−xTex and SrSe1−xTex ternary alloys. For exchange–correlation energy and corresponding potential, the generalized gradient approximation (GGA) by Perdew–Burke–Ernzerhof (PBE) and Engel–Vosko (EVGGA) have been used. Deviation of the lattice constants from Vegards law and the bulk modulus from linear concentration dependence (LCD) were observed for the three alloys. The microscopic origins of the gap bowing were explained by using the approach of Zunger and co-workers. The refractive index and optical dielectric constant for the alloys of interest are calculated by using different models. In addition the thermodynamic stability of the alloys was investigated by calculating the critical temperatures of alloys.