Observation of two kinds of phase transitions of ZrW2O8 by power-compensated differential scanning calorimetry and high-temperature X-ray diffraction

Two kinds of phase transitions were observed for ZrW2O8 at 123 and 164°C with power-compensated differential scanning calorimetry. An endothermic peak was observed at 123°C in the DSC curve, indicating that the phase transition at this temperature is of the first order. The variation of enthalpy, ΔH, at the phase transition was estimated to be ∼290 J/mol. The base-line shift of the DSC curve, corresponding to the second-order phase transition, was observed at 164°C. The change in the heat capacity, ΔCp, at 164°C was calculated to be ∼20 J/(mol K), showing good agreement with the result of adiabatic calorimetry. The X-ray diffraction patterns of ZrW2O8 below 120°C and above 164°C could be explained by using models with space groups of P213 (No. 198) and Pa3̄ (No. 205), respectively. Changes of the linear thermal expansion coefficients were observed at around 123 and 164°C, showing good agreement with the phase transition temperatures measured with DSC.