Phase Transitions and Thermodynamic Properties of Rubidium and Cesium Cyanides

Heat-capacity measurements were performed for rubidium and cesium cyanide crystals between 15 and 300°K. It is found that each crystal has only one anomalous peak in its heat capacity curve, being located at 110.3°K and 193.1°K for rubidium and cesium cyanides, respectively. By the comparison of the present data with those for potassium and sodium cyanides, it was shown that these anomalous heat capacities can be interpreted in terms of the orientational ordering processes of cyanide ions in the crystals. It was suggested that there should exist the residual entropy due to the frozen-in state in their orientational configuration and they were estimated as Rln4 for rubidium cyanide and Rln2 for cesium cyanide.