Dielectric theory of strained molecular crystals: Elasto-optic coefficients of orthorhombic sulphur

The general dielectric theory of molecular crystals is extended to calculate the strain derivatives of dielectric properties, assuming a fixed polarizability relative to the molecular axes. The molecular rotation due to strain makes an important contribution, for which an empirical prescription is given. To illustrate the theory, the elasto-optic coefficients for orthorhombic sulphur are calculated. They apree well with experiment when each sulphur atom is treated as a polarizable point. It is concluded that the polarizability can be treated as fixed, for example in deducing changes of molecular orientation from birefringence measurements through phase transitions.