Ab initio calculation of the elastic properties and the lattice dynamics of the AgBr1−xClx alloy

The lattice dynamics and elastic properties of the ternary AgBr1−xClx alloy have been studied using first-principles calculations. These are done using the density-functional perturbation theory (DFPT) and employing the virtual-crystal approximation (VCA). We study the variation of the optical phonon frequencies (ωTO and ωLO) and of the acoustical phonon frequencies (ωTA and ωLA), the high-frequency dielectric coefficient (ε∞), the dynamic effective charge (Z∗) and the elastic constants (C11, C12, C44) as a function of the composition (x). We find that the phonon frequencies as well as the elastic constants follow a quadratic law in x and agree well with the available experimental results. The elastic constant tensors for simple cubic 8-atom supercells with x = 0.25, 0.5 and 0.75 have been computed and are in good agreement with those obtained from VCA and experiments.