The specific heats of some alkali halides at very low temperatures

The specific heats of KI, NaI and KBr have been measured in the temperature range 0.5 to 1.5 °K and the low temperature limiting Debye temperatures found to be 133.1 ± 1.5 °K, 164.3 ± 1.5 °K and 173.8 ± 1.5 °K respectively. The results are compared with those of other workers and agree, within the combined experimental error (less than 2%), with Debye temperatures calculated from low temperature elastic constant measurements. Results of previous work on LiF are also discussed and there is shown to be excellent agreement between the calorimetric and elastic Debye temperatures. The present measurements were made by means of a novel type of calorimeter in which spring loaded `feet' are used to establish thermal contact with the crystals and thus overcome difficulties arising from large differential thermal contractions. A previously unsuspected source of error, which may occur in calorimetry when superconducting heat switches are employed, was revealed and is analysed.