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 degrees C with power-compensated differential scanning calorimetry. An endothermic peak was observed at 123 degrees C in the DSC curve, indicating that the phase transition at this temperature is of the first order. The variation of enthalpy, Delta H, at the phase transition was estimated to be similar to 290 J/mol. The base-line shift of the DSC curve, corresponding to the second-order phase transition, was observed at 164 degrees C. The change in the heat capacity, Delta C-p, at 164 degrees C was calculated to be similar to 20 J/(mol K), showing good agreement with the result of adiabatic calorimetry. The X-ray diffraction patterns of ZrW2O8 below 120 degrees C and above 164 degrees C could be explained by using models with space groups of P2(1)3 (No. 198) and Pa (3) over bar (No. 205), respectively. Changes of the Linear thermal expansion coefficients were observed at around 123 and 164 degrees C, showing good agreement with the phase transition temperatures measured with DSC. (C) 2000 Elsevier Science Ltd. All rights reserved.