Thermal Analysis of Phase Transition in Negative-Thermal-Expansion Oxide, ZrW_2O_8

摘要

Differential scanning calorimetry (DSC) curves of negative-thermal-expansion oxide, ZrW_2O_8, were obtained under dry N_2 atmosphere. An endothermic peak and base-line shift were observed at 124 and 169 deg C, respectively, upon heating measurements of as-prepared ZrW_2O_8, Upon successive cooling and heating measurements, the base-line shift at 169 deg C was reproduced, however, the peak at 124 deg C disappeared in both curves. The base-line shift at 169 deg C coresponded to the variation of the thermal expansion coefficient due to a structural phase transition from the acentric cubic phase with the space group of P2_13 (No. 198) to the centric phase with the space group Pa3(No. 205). Thermogravimetry (TG) and termogravimetry differential thermal analysis mass spectroscopy (TG-DTA-MS) simultaneous measurement revealed that the origin of the endothermic peak at 124 deg C was the secession of H_2O involved with the ZrW_2O_8 specimen, whose amount was less than 1.6 mol%. It was clarified that the trace amount of H_2O negligibly affected the thermal expansion behavior of ZrW_2O_8 and that the variation of the thermal expansion coefficient around 164 deg C could be attributed to the gamma-type transition.