Phase transition and glass transition in rubidium cyanide

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Abstract
Heat capacity of a pure rubidium cyanide crystal has been measured between 14 and 330 K with an adiabatic calorimeter. A first order phase transition was found at (132.30±0.05) K in agreement with the previous dielectric study. The transition enthalpy and entropy are 1280 J mol− 1 and 10.2 J K− 1 mol− 1, respectively, of which (742±2) J mol− 1 and (5.65±0.02) J K− 1 mol− 1 are the discontinuous parts. Under the assumption that the orientational disorder of the cyanide ion in the cubic phase is eightfold, a residual entropy of 5.43 J K− 1 mol− 1 was derived from the transition entropy and related to the head‐to‐tail disorder of the anions. The disorder causes a glass transition at 30 K. The enthalpyrelaxation time determined by spectroscopic use of the adiabatic calorimeter agrees very well with the dielectric relaxation time extrapolated to the lower temperature, giving evidence for the frozen disordered state to be a dipolar glass.