Photoelasticity of crystals with the scheelite structure: quantum mechanical calculations

We report a complete set of elastic, piezooptic and photoelastic tensor constants of scheelite crystals CaMoO₄, BaMoO₄, BaWO₄ and PbWO₄ determined by density functional theory (DFT) calculations using the quantum chemical software package CRYSTAL17. The modulation parameter, i.e. the change in the crystal optical path normalized by thickness and mechanical stress, was calculated based on piezooptic and elastic compliance tensor constants. For the geometries of the most effective piezo-optic interactions, this parameter reaches rather large values (16-17) × 10^-12 m² N^-1. Anisotropy of the photoelastic and acoustooptic effects is explored by means of indicative surfaces, considering the directions of light propagation and polarization, the direction of uniaxial compression or lattice distortion caused by the propagation of the acoustic wave. DFT calculations indicate BaWO₄ and PbWO₄ crystals as the most effective acousto-optic materials, predicting the figure of merit constant M₂ ∼ 20 × 10^-15 s³ kg^-1. The methodology proposed combines the DFT calculations and photoelasticity caused by uniaxial compression of the crystal lattice, with particular emphasis on its anisotropy. It can be considered as part of optical engineering aimed at preliminary assessment of the photoelastic properties of crystal materials, thus assisting in their selection for synthesis and relevant applications.