Thermal analysis of phase transition in negative-thermal-expansion oxide, ZrW2O8 -: Detection of trace amount of H2O and λ-type transition

Differential scanning calorimetry (DSC) curves of negative-thermal-expansion oxide, ZrW2O8, were obtained under dry N-2 atmosphere. An endothermic peak and base-line shift were observed at 124 and 169degreesC, respectively, upon heating measurements of as-prepared ZrW2O8. Upon successive cooling and heating measurements, the base-line shift at 169degreesC was reproduced, however, the peak at 124degreesC disappeared in both curves. The base-line shift at 169degreesC corresponded 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(1)3 (No. 198) to the centric phase with the space group of Palpha(3) over bar (No. 205). Thermogravimetry (TG) and thermogravimetry differential thermal analysis mass spectroscopy (TG-DTA-MS) simultaneous measurement revealed that the origin of the endothermic peak at 124degreesC was the secession of H2O involved with the ZrW2O8 specimen, whose amount was less than 1.6 mol%. It was clarified that the trace amount of H2O negligibly affected the thermal expansion behavior of ZrW2O8 and that the variation of the thermal expansion coefficient around 164degreesC could be attributed to the A-type transition.