Phonon scattering from substitutional impurities and lattice defects in silver chloride

Abstract

The purpose of this investigation was to study phonon scattering by substitutional impurities and lattice defects in silver chloride crystals by means of the thermal conductivity technique. In addition, far infrared absorption measurements were made on some of the doped crystals. The thermal conductivity of silver chloride as a function of the concentrations of eight impurities-- Li+, Na+, K+, Rb+, uC +, Cu + + , Br - and I---was measured in the temperature range 1.2°K to 80 0 K. Only in the quenched heavily doped lithium crystal was a definite therma1 conductivity dip observed. The strong low temperature depressions in the thermal conductivity of the peavi1y doped Li+ and Cu++ doped crystals suggest possib1e resonance below 10 K. The far infrared absorption of some of the doped crystals was measured from 33 cm -1 to 105 cm -1 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 any induced absorption. The thermal conductivity of pure silver chloride crystals, which were subjected to various heat and surface treatments, was measured. The results suggest that the phonon scattering is mainly due to dislocations and that diffusive boundary scattering is perhaps due to the dynamical dislocation loops beneath the surface. The pure single crystal thermal conductivity curve was fitted by using the Debye thermal conductivity integral. Due to 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. More information about the phonons in the silver chloride lattice is needed in order to perform significant analysis on the far infrared absorption spectra.