Phonon Scattering by Point Defects in Ca${\mathrm{F}}_2$, Sr${\mathrm{F}}_2$, and Ba${\mathrm{F}}_2$

Low-temperature thermal-conductivity measurements have been made on pure Ca${\mathrm{F}}_2$, Sr${\mathrm{F}}_2$, and Ba${\mathrm{F}}_2$, and on all three hosts containing ${\mathrm{Li}}^{+}$ or ${\mathrm{Na}}^{+}$ impurities. Concentrations of these impurities near ${10}^{18}$ ${\mathrm{cm}}^{-3\mathrm{}}$ were found to have no effect on the thermal conductivity, suggesting a weak phonon scattering due to point defects in these systems. ${\mathrm{Sm}}^{++}$ and ${\mathrm{U}}^{3+}$ impurities in the ${10}^{19}$-${\mathrm{cm}}^{-3\mathrm{}}$ range and ${\mathrm{H}}^{-}$ in the ${10}^{20}$-${\mathrm{cm}}^{-3\mathrm{}}$ range were studied in Ca${\mathrm{F}}_2$, and two effects were observed. Nonresonant point-defect scattering of the Rayleigh type was seen. Mass-difference scattering alone was shown to be sufficient to explain the data for all three impurities. The strength of this Rayleigh scattering is in accord with the lack of an effect at low concentrations of ${\mathrm{Li}}^{+}$ and ${\mathrm{Na}}^{+}$. In addition, the low-temperature side of the conductivity curves for all three dopants was found to vary as $T^3$. This behavior has been ascribed to phonon scattering by precipitates. Since ${\mathrm{Sm}}^{++}$ and ${\mathrm{U}}^{3+}$ are magnetic, the possibility of phonon-spin interactions for these systems was investigated as an explanation for the low-temperature data. This possibility was shown to be excludable.