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 (Θ D ) have been calculated for all of the experi­mental results. The variation of Θ D with temperature is in general accord with the predictions of theory. Starting from the lowest temperatures, Θ D for the three potassium salts first decreases to a minimum in the region of T = Θ D /12, then increases to a maximum in the region of T = Θ D /2 and subsequently decreases smoothly. Θ D for sodium iodide shows the same behaviour at low temperatures but is still increasing at 270° K.