Photoelasticity of crystalline and amorphous silica from first principles

Based on density-functional perturbation theory we have computed from first principles the photoelastic tensor of few crystalline phases of silica at normal conditions and high pressure (quartz, α-cristobalite, and β-cristobalite) and of models of amorphous silica (containing up to 162 atoms), obtained by quenching from the melt in combined classical and Car-Parrinello molecular-dynamics simulations. The computational framework has also been checked on the photoelastic tensor of crystalline silicon and MgO as prototypes of covalent and ionic systems. The agreement with available experimental data is good. A phenomenological model suitable for describing the photoelastic properties of different silica polymorphs is devised by fitting it on the ab initio data.