First principle calculations of structural, electronic, thermodynamic and optical properties of Pb1−xCaxS,Pb1−xCaxSe and Pb1−xCaxTe ternary alloys

Using first principles total energy calculations within the full potential linearized augmented plane wave (FP-LAPW) method, we have investigated the structural, electronic, thermodynamic and optical properties of Pb1−xCaxS, Pb1−xCaxSe and Pb1−xCaxTe ternary alloys. The effect of composition on lattice parameter, bulk modulus, band gap, refractive index and dielectric function was investigated. Deviations of the lattice constants from Vegard's law and the bulk modulus from linear concentration dependence were observed for the three alloys. Using the approach of Zunger and co-workers, the microscopic origins of band gap bowing have been detailed and explained. The disorder parameter (gap bowing) was found to be mainly caused by the chemical charge transfer effect. On the other hand, the thermodynamic stability of these alloys was investigated by calculating the excess enthalpy of mixing, ΔHm, as well as the phase diagram. It was shown that all of these alloys are stable at low temperature. The calculated refractive indices and optical dielectric constants were found to vary nonlinearly with Ca composition.