Theory of Infrared Optical Properties of Fluorite Crystals

This paper provides a theoretical background for, and some interpretation of, recent measurements of the infrared optical properties of fluorite crystals particularly Ca${\mathrm{F}}_2$, Sr${\mathrm{F}}_2$, Ba${\mathrm{F}}_2$, and Cd${\mathrm{F}}_2$. A quantum-mechanical derivation of the optical properties in the harmonic approximation is given. The most reasonable procedure for the phenomenological treatment of damping effects is described. The physical origin of damping effects is discussed and it is shown how the perturbation theoretic results may be simplified in the low-frequency limit. Damping functions for the above materials are presented as a function of frequency and temperature.