Long-Wavelength Optical Lattice Vibrations in BaySr1−yF2 and SryCa1−yF2

Abstract

The far-infrared reflectivity spectra of ${\mathrm{Sr}}_y{\mathrm{Ca}}_{1-y}{\mathrm{F}}_2$ and ${\mathrm{Sr}}_y{\mathrm{Ba}}_{1-y}{\mathrm{F}}_2$ have been measured both at room temperature and at 100°K over the spectral range from 10 to 100 μ. Chang et al. have observed a single Raman mode in these crystals and suggested that a virtual-ion model is applicable. This is to be distinguished from such systems as $\mathrm{Ga}{\mathrm{As}}_y{\mathrm{P}}_{1-y}$ and $\mathrm{Cd}{\mathrm{Se}}_y{\mathrm{S}}_{1-y}$ where the appearance of two reststrahlen bands leads to a more complicated model. Even though the reflectivity spectra of these mixed fluorides show essentially a single reststrahlen band, these bands have some fine structure. The infrared reststrahlen band shifts smoothly but in a nonlinear manner with $y$. The model used previously for $\mathrm{Ga}{\mathrm{As}}_y{\mathrm{P}}_{1-y}$ is adapted to the present case and is shown to predict both the Raman and infrared spectra including the fine structure. The model shows that the virtual-ion approximation is valid for the main Raman mode but not for the infrared modes. Contrary to the case of $\mathrm{Ga}{\mathrm{As}}_y{\mathrm{P}}_{1-y}$, no significant clustering of like cations around anions is detected.