Thermal Conductivity of Some Alkali Halides Containing Divalent Impurities. I. Phonon Resonances

Abstract

Thermal-conductivity measurements have been made on KCl, KBr, and NaCl containing divalent impurities. In all cases dips in the plots of thermal conductivity versus temperature are observed in the temperature range 25-50°K. The data are understood in terms of a model due to Krumhansl, in which resonant phonon scattering due to changed force constants is considered. Within the framework of this model, resonant frequencies of the order of 40-90 ${\mathrm{cm}}^{-1\mathrm{}}$, depending on the host lattice, are deduced. For crystals where the divalent impurity is the same size as the host ion, the scattering is postulated as being caused largely by the accompanying positive-ion vacancy. When the divalent ion has a size different from the host, the situation is more complicated and cannot be simply understood.