Thermal Conductivity of Some Alkali Halides Containing Divalent Impurities. II. Precipitate Scattering

Abstract

The lattice thermal conductivity of quenched and annealed KCl and KBr containing ${\mathrm{Ba}}^{++}$ or ${\mathrm{Sr}}^{++}$ has been measured over the temperature range of 0.35-10°K. Quenched crystals have a conductivity which one expects for phonon scattering by precipitates about 100 Å in size, being Rayleigh-like at lowest temperatures and geometrical at higher temperatures. Annealed crystals have a conductivity which is a mixture of precipitate, dislocation, and stacking-fault scattering of phonons. The precipitate scattering shows up best in annealed crystals when the precipitate's crystal structure most nearly matches the host's. If the mismatch is large, then dislocation-type scattering dominates. It is proposed that precipitation of divalent impurities in alkali halides occurs in two stages. In the early stages precipitates form at selected regions of the crystal, are about 100 Å in size, and scatter phonons as one would expect they should. In the late stages, when many of the divalent ions are out of solid solution, the precipitates are larger and the dislocations and stacking faults surrounding them dominate the phonon scattering.