The Effect of Point Imperfections on Lattice Conduction in Solids

The thermal conductivity of single crystals of LiF containing various relative concentrations of the isotopes 6Li and 7Li has been measured between 10 and 90 °K. An increase in thermal resistance with isotope concentration (up to 50% of either isotope) is observed, but the results at 30 °K fit neither the formula of Klemens (1955) nor the formula previously deduced for the limit of small isotope concentrations (Berman, Foster & Ziman 1956). In a new variational treatment a more general trial function is used and the contribution of the phonon-phonon iV-processes is calculated explicitly. This gives results in good agreement with the present experiments and also with published observations on a wide range of crystals containing known concentrations of point imperfections.