Thermal expansion at low temperatures III. potassium chloride

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Abstract
Observations of the linear thermal expansion of a potassium chloride crystal have been made between 2° and 30°k, 55° and 90°k, and 273° and 283°k. At the lowest temperatures (3° < T < 9°k). the linear coefficient of expansion α ≃ 5·4 ± 0·3 × 10−11 T b per °c, which corresponds to a Grüneisen γ value of 0·39 ± 0·2. γ increases with temperature, particularly in the range 15° to 70°k (cf. θD≃ 235°k), being about 0·9 at 30°k, 1·25 (60°k) and 1·5 above 200°k. This increase is of the form suggested theoretically by Barron and by Blackman for ionic cubic solids but is greater than they expected. It is not very different from that predicted by Sheard from the pressure variation of the elastic constants.