Phonon Scattering from Substitutional Impurities and Lattice Defects in Silver Chloride

Abstract

The thermal conductivity of silver chloride doped with varying concentrations of eight impurities—${\mathrm{Li}}^{+}$, ${\mathrm{Na}}^{+}$, ${\mathrm{K}}^{+}$, ${\mathrm{Rb}}^{+}$, ${\mathrm{Cu}}^{+}$, ${\mathrm{Cu}}^{++}$, ${\mathrm{Br}}^{-}$, and ${\mathrm{I}}^{-}$—was measured in the temperature range 1.2 to 80°K. Only in the quenched, heavily lithium-doped crystal was a definite thermal conductivity dip observed. The strong low-temperature depressions in the thermal conductivity of the ${\mathrm{Cu}}^{++}$-doped crystals suggest a possible resonance below 1°K. The far-infrared absorption of some of the doped crystals was measured from 33 to 105 ${\mathrm{cm}}^{-1\mathrm{}}$ at 7°K. Impurity-induced absorption peaks were observed in all cases, except for the rubidium-doped crystal, where the concentration was perhaps too low to give rise to an induced absorption. The thermal conductivity of pure silver-chloride crystals subjected to various heat and surface treatments was also measured. The results suggest that phonon scattering is mainly due to dislocations and that diffuse boundary scattering is perhaps due to dynamical dislocation loops beneath the surface. The thermal conductivity curve of the pure single crystal was fitted by using the Debye thermal conductivity integral. Because of the existence of many low-energy critical points, a two-group-velocity model was employed. A relatively strong normal-process relaxation rate suggests that anharmonicity may be important.