Influence ofFCenters on the Lattice Thermal Conductivity in LiF

Abstract

The influence of photochemically produced $F$ centers in LiF on the thermal conductivity is investigated, the density of the $F$ centers being determined optically. The $F$ centers decrease the thermal conductivity appreciably at low temperatures. Additive coloration of KCl has a similar effect. The experimental results are quite different from the results obtained in the case of the isotope effect in Ge. Callaway's theory for the lattice thermal conductivity in the presence of point defects cannot explain the present observations satisfactorily, although it is superior to Klemens'. Two explanations are proposed: (1) The $F$ centers are not randomly distributed (but the presence of clusters of $F$ centers can be excluded in the experiments). (2) The long-range strain field around the $F$ center does not scatter like a point defect. In support of Callaway's theory, it is shown that his model can explain an experimental observation (first reported by Toxen) about the influence of point defects on the thermal conductivity.