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.
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Berman et al. (1959) studied this question.
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