Phonon scattering by point defects in CaF2, SrF2, and BaF2

Low-temperature thermal-conductivity measurements have been made on pure CaF2, SrF2, and BaF2, and on all three hosts containing Li+ or Na+ impurities. Concentrations of these impurities near 1018 cm-3 were found to have no effect on the thermal conductivity, suggesting a weak phonon scattering due to point defects in these systems. Sm++ and U3+ impurities in the 1019-cm-3 range and H- in the 1020-cm-3 range were studied in CaF2, 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 Li+ and Na+. In addition, the low-temperature side of the conductivity curves for all three dopants was found to vary as T3. This behavior has been ascribed to phonon scattering by precipitates. Since Sm++ and U3+ 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.