The Heat Capacity of CuInP2Se6, a Layer-Structured Selenodiphosphate

The heat capacity of CuInP 2 Se 6 has been measured at temperatures between 5 and 350 K. A lambda anomaly occurred between 150 and 250 K with the peak at 225–230 K. The excess heat capacity was determined by subtracting the phonon contribution from the experimental heat capacity taking into account the distribution of the phonon frequencies and Cp-Cv difference by the least squares method. The transition enthalpy thus determined is 2250 J mol−1 and the transition entropy 11.1 J K−1 mol−1 . These values are discussed in relation to the positional disorder of the copper (I) ions and the interaction among them.