Scattering of Phonons by Monatomic Impurities in Potassium Halides

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 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 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 Li⁺- and 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.