Thermal Properties of Alkali Halide Crystals with the CsCl-type Structure. I. Construction of a Calorimeter for the Temperature Region 1–20°K and Heat Capacities of Caesium Bromide and Caesium Iodide between 1.5 and 300°K

Abstract

Abstract An adiabatic low-temperature calorimeter, capable of measuring the heat capacities of solid or liquid between 1.4 and 21.0°K, was constructed. A mechanical thermal switch is used for cooling the sample cell, the temperature of which is measured with a doped germanium thermometer. The resistance-temperature calibration data for the resistor was fitted to a function of the form log10R=A⁄T+B+CT+DT2. By using this formula the thermodynamic temperature scale is represented to within ±0.02°K and by combining the correction curves the final deviations of the calibration data from the thermodynamic temperature scale were within ±0.003°K in the whole temperature range investigated. The heat capacity measurements of CsBr and CsI crystals were made with two kinds of adiabatic calorimeter assemblies, one for the temperature range 1.4 to 21°K and the other for the range 12 to 300°K. Values of the heat capacity Cp°, entropy S°, enthalpy function (H°−H0°)/T, and Gibbs energy function −(G°−H0°)/T at 298.15°K are: 12.441, 26.785, 10.415, and 16.370 for CsBr and 12.571, 29.127, 10.788, and 18.339 for CsI, respectively, in cal·mol−1.°K−1.