Strain-Induced Splitting and Oscillator-Strength Anisotropy of the Infrared Transverse-Optic Phonon in Calcium Fluoride, Strontium Fluoride, and Barium Fluoride

Strain-induced splittings of the infrared-active TO phonon in Ca${\mathrm{F}}_2$, Sr${\mathrm{F}}_2$, and Ba${\mathrm{F}}_2$ are calculated from a least-squares fit of photoelastic dispersion data by an undamped oscillator model. The strain-induced oscillator-strength anisotropy appears negligible; the Szigeti effective charge remains a scalar. The shear deformation potential for [100] strain is ∼170 ${\mathrm{cm}}^{-1\mathrm{}}$ in all the materials; for [111] strain it is -82 ${\mathrm{cm}}^{-1\mathrm{}}$ for Ca${\mathrm{F}}_2$, -54 ${\mathrm{cm}}^{-1\mathrm{}}$ for Sr${\mathrm{F}}_2$, and -50 ${\mathrm{cm}}^{-1\mathrm{}}$ for Ba${\mathrm{F}}_2$.