Thermodynamic Properties and Order-Disorder Phase Transitions of the Potassium Cyanide Crystal

The heat capacities of the potassium cyanide crystal have been measured over the temperature range from 13 to 310°K by using a low temperature adiabatic calorimeter. It has been found that there are two regions of anomalous heat capacity, with maxima at 82.9±0.2°K and 168.3±0.1°K respectively. The heats and entropies of transition have been evaluated by utilizing a ΘD-versus-temperature plot. The entropy of the lower transition amounts to about R ln 2, which means that the transition is due to the orientational order-disorder rearrangement of the cyanide ion with respect to its head and tail. A perfectly-ordered structure for the orientation of the cyanide ions has been suggested. A certain small anomaly in heat capacity has been found around 190°K, one which seems to be associated with some change in higher-order structural defects created by the upper transition phenomenon. The torsion-rotational heat capacity of the cyanide ions has been separated from the total on some reasonable assumptions. The results show that the cyanide ions below the lower transition point are subjected to torsional oscillation with the wave number of about 160 cm−1, a value which corresponds to the barrier height of 7.5 kcal. mol−1 based on a simple harmonic oscillator model. On the other hand, they are in a hindered rotational state with a barrier height of approximately 1.35 kcal. mol−1 in the cubic phase, the barrier height diminishing from 7.5 kcal. mol−1 gradually in a single phase or abruptly via the transition points. The thermodynamic functions for this crystal have been evaluated by integrating the heat capacity data. The values at 298.15°K are, in units of cal.mol−1deg−1: Cp°=15.84, S°=30.71, (H°–H°O)⁄T=13.94 and –(G°–H°O)⁄T=16.77 respectively.