Scattering of Phonons by Monatomic Impurities in Potassium Halides

Abstract

The thermal conductivity of doped KCl, KBr, and KI has been measured between 1 and 200°K. Below ∼5°K, the phonon scattering by the defects obeys a Rayleigh law in most cases. The magnitude of the scattering is determined by the mass difference alone. In ${\mathrm{Ag}}^{+}$-doped crystals, however, the Rayleigh scattering is much weaker than predicted by the mass difference. This is explained through the weak coupling between the ${\mathrm{Ag}}^{+}$ and its neighbors. Above 5°K, in the region of the resonance scattering, the conductivity can be qualitatively described with the model of elastic phonon resonant scattering by quasilocalized modes. In KCl:Tl, I, Rb, and Br, the mass difference alone suffices to qualitatively describe the data, which means that the resonant mode is odd. The resonant scattering observed in ${\mathrm{Li}}^{+}$- and ${\mathrm{Ag}}^{+}$-doped crystals is ascribed to excitations of even resonant modes. It is noted that in many cases thermal-conductivity measurements are complementary to optical-absorption measurements, since the former observe defects strongly coupled to the lattice, whereas the latter observe the ones which are weakly coupled.