Model for the pressure-optic coefficients in optical materials

A physically meaningful Sellmeier-type equation is formulated to account for the change in refractive index against pressure and its dispersion, which includes exactly the contributions of the isothermal compressibility, the isobaric (electronic and lattice) band gaps, and the pressure coefficients of the excitonic and lattice resonance absorption band gaps. It allows one to interpret the measured values satisfactorily in terms of the optical band gaps instead of on consideration of the molar polarizability.