The thermal expansion of alkali halides at low temperatures

Linear thermal expansions of eight alkali halides have been determined at liquid oxygen temperatures and at temperatures from 30 °K down to 2 °K. For temperaturesT≤ θ/20, where θ is the Debye temperature, the expansion coefficients are well represented by α =AT3+BT5. Values are reported for the Grüneisen parameter γ = 3αV/Cx, whereC/Vis the heat capacity per unit volume and x is the compressibility. For CsBr (b.c.c. structure) γ appears to be nearly independent of temperature, with a value of 2·0 but for the other crystals, which have the rock-salt structure, the parameter γ varies with temperature, chiefly betw een θ/10 and θ/5. At room temperature, γ lies between 1·45 and 1·7 but at low temperature this generally decreases to a value γ0which isca.—0·1 for RbI, +0·3 for KCl, KBr and KI and 1·0 for NaCl and Nal; LiF does not show this decrease, γ0being 1·7. The values observed for γ0are compared with those calculated from elastic constants and their pressure derivatives and the general behaviour of γ(T) is observed to conform qualitatively to the predictions of simple theoretical models of Born, Blackman and Barron.