First principle investigation of AlAs and AlP compounds and ordered AlAs1−xPx alloys

We have investigated the structural and electronic properties of AlAs and AlP compounds and of ordered AlAs1−xPx alloys using the full potential linearized augmented plane wave plus local orbitals (FP-LAPW + lo) method based on density functional theory. The total energies and structural quantities of those compounds have been calculated for different approximations of exchange–correlation energy. The electronic quantities have been found to be in good agreement with the corresponding measured ones when the compounds were defined by the lattice constants of Perdew–Wang-generalized gradient approximation (PW-GGA) scheme. The PW-GGA approach was also applied on ordered AlAs1−xPx alloys to study the effect of composition on lattice constant, band gap, and refractive index of AlAs1−xPx ternary alloys. The calculated lattice constants scale linearly with composition (Vegard’s law). The microscopic origins of the gap bowing were explained by using the approach of Zunger and co-workers. In addition to (FP-LAPW + lo) method, the composition dependence of the refractive index was studied by Reedy and Nazeer model. The thermodynamic stability of these alloys was investigated by calculating the excess enthalpy of mixing ΔHm as well as the phase diagram.