The thermal expansion of alkali halides at low temperatures

Abstract

Linear thermal expansions of eight alkali halides have been determined at liquid oxygen temperatures and at temperatures from 30 $^\circ$K down to 2 $^\circ$K. For temperatures T $\leqslant \theta$/20, where $\theta$ is the Debye temperature, the expansion coefficients are well represented by $\alpha$ = AT$^3$ + BT$^5$. Values are reported for the Gruneisen parameter $\gamma$ = 3$\alpha V$/C$\chi$, where C/V is the heat capacity per unit volume and $\chi$ is the compressibility. For CsBr (b.c.c. structure) $\gamma$ 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 $\gamma$ varies with temperature, chiefly between $\theta$/10 and $\theta$/5. At room temperature, $\gamma$ lies between 1.45 and 1.7 but at low temperature this generally decreases to a value $\gamma_0$ which is ca. -0.1 for RbI. +0.3 for KCl. KBr and KI and 1.0 for NaCl and NaI; LiF does not show this decrease, $\gamma_0$ being 1.7. The values observed for $\gamma_0$ are compared with those calculated from elastic constants and their pressure derivatives and the general behaviour of $\gamma$(T) is observed to conform qualitatively to the predictions of simple theoretical models of Born. Blackman and Barron.