Infrared Properties of Ca${\mathrm{F}}_2$, Sr${\mathrm{F}}_2$, and Ba${\mathrm{F}}_2$

Reflectivity and transmission measurements on Ca${\mathrm{F}}_2$, Sr${\mathrm{F}}_2$, and Ba${\mathrm{F}}_2$ have been studied to obtain information on the lattice vibrations. The reflection band in each material has been analyzed with classical dispersion theory. In order to fit the data, it was necessary to use one strong and one weak resonance in each case. The strong resonance is identified as the transverse optical mode of vibration for zero wave vector and is at 38.9, 46.1, and 54.3 μ for Ca${\mathrm{F}}_2$, Sr${\mathrm{F}}_2$, and Ba${\mathrm{F}}_2$, respectively. The long-wavelength value for the dielectric constant obtained from the optical measurements indicated a disagreement with the published value for Sr${\mathrm{F}}_2$ obtained from capacity measurements. Capacity measurements were made and showed the previously accepted value for Sr${\mathrm{F}}_2$ to be in error. The transverse optical-mode frequencies are shown to agree very well with the spacing of some of the satellite lines in the emission spectrum of ${\mathrm{Sm}}^{++}$ in these fluorides.