Phonon Optical Properties ofCa1−xSrxF2

The phonon optical properties of the mixed fluorite system ${\mathrm{Ca}}_{1-x}{\mathrm{Sr}}_x{\mathrm{F}}_2$ are treated theoretically using the "average-Green's-function" formalism. The calculations are based on a rigid-ion model for the pure Ca${\mathrm{F}}_2$ lattice, with a defect model for ${\mathrm{Sr}}^{++}$ impurities that includes both mass and force-constant changes. Results are compared with experimental observations on Raman scattering and far-infrared reflectivity spectra.