Thermal Expansion of Alkali Halides at Low Temperatures

Abstract

The linear thermal expansions of LiF, NaCl, KCl, KBr and KI single crystals have been measured by a Fizeau interferometric method in the temperature range 20° to 270 °K with an accuracy to 0.04 × 10^-5 deg^-1. The apparatus is described, and the expansion coefficients as a function of temperature are shown graphically together with those of previous workers. Smoothed values of the volume expansion coefficients β, and values of the Grüneisen parameter γ, calculated from β using recent experimental elasticity and specific heat data, are tabulated. For all except LiF, the values of γ show a marked decrease towards low temperatures in qualitative agreement with simple point-charge ion models. For LiF, however, γ appears to be constant (= 1.60) to within 3% over the whole temperature range.