Calorimetric study on phase transition of lanthanum perchlorate octahydrate

Heat capacities of lanthanum perchlorate octahydrate La(ClO₄)₃8H₂O have been measured from 11 to 320 K by an adiabatic calorimeter. Two phase transitions were observed at 115.5 K and 285.2 K, respectively. The enthalpy and entropy of the upper phase transition amounted to 17.97 kJ mol^?1 and 62.98 JK^?1mol^?1. Anomalously large entropy change can be interpreted by a statistical model including a combined positional and orientational disorder of the hydrate water molecules, in accord with an unusually high crystal symmetry of the room temperature phase.