The thermal properties of alkali halide crystals. I. The heat capacity of potassium chloride, potassium bromide, potassium iodide and sodium iodide between 2.8 and 270˚K

Abstract The measurements were made with two adiabatic calorimeter assemblies, one for the temperature range 2.8 to 20°K and the other for the range 12 to 270°K. The working thermometer for the former assembly was a carbon resistor and for the latter a strain-free capsule-type platinum resistance thermometer. The crystals (pieces approximately 0.5 cm in dimension) were sealed in thin copper calorimeter vessels with a small amount of helium gas to promote heat exchange. The accuracy of the measurements is estimated to be ± 2% at the lowest temperatures, rising rapidly to ± 0.2% for T ⋝ 20°K. The precision was generally better than this by about a factor of two at most temperatures. Apparent Debye characteristic temperatures (ΘD) have been calculated for all of the experi­mental results. The variation of ΘD with temperature is in general accord with the predictions of theory. Starting from the lowest temperatures, ΘD for the three potassium salts first decreases to a minimum in the region of T = ΘD/12, then increases to a maximum in the region of T = ΘD/2 and subsequently decreases smoothly. ΘD for sodium iodide shows the same behaviour at low temperatures but is still increasing at 270° K.