Thermal Conductivity of Some Alkali Halides ContainingFCenters

Thermal conductivity measurements are used to study phonon scattering by F centers in NaF, KCl, KBr, and KI. F centers were produced by additive coloration and by x and γ irradiation. The total effect on the thermal conductivity is seen to be different for the two methods of preparation. The additively colored data display two features: a low-temperature depression due to phonon scattering by colloids and quench-induced defects, and a high-temperature effect due to phonon scattering by F centers. The irradiated samples also display two features, a high-temperature effect due to F centers and a different low-temperature depression of unknown origin. The high-temperature data in irradiated samples can be explained if one assumes that F centers are formed in regions of high concentration during irradiation. Several mechanisms are discussed for the low-temperature depression in irradiated crystals, including interstitials and the effect of dislocations on F-center formation. The F center is shown to produce quasilocalized impurity modes with frequencies of the order of 10¹³ sec^-1, and is seen to scatter phonons elastically primarily through its strain field.