Phase behaviors of the ZrMo2-xWxO8 (x = 0.2-2.0) system and the preparation of an Mo-rich cubic phase

The phase behaviors of ZrMo 2 - x W x O 8 (x = 0.2 to 2.0) solid solutions have been studied systematically by variable-temperature X-ray diffraction (XRD) and differential scanning calorimetric (DSC) techniques. A low-temperature (LT) orthorhombic ZrMo 2 - x W x O 8 (x = 0.2-2.0) phase is formed by dehydration of the precursors ZrMo 2 - x W x O 7 (OH) 2 (H 2 O) 2 (x = 0.2-2.0) within a temperature range from 124 to 203 °C. Among them, the orthorhombic ZrMo 2 - x W x O 8 solid solutions are able to convert further into metastable cubic phases within an x-dependent temperature range from 502 to 552 °C; the trigonal phases become dominant at higher temperature for Mo-rich solid solutions. However, no trigonal phases are formed in the high-temperature region for W-rich solid solutions. The absence of decomposition of ZrMo 2 - x -W x O 8 (x = 0.2 to 1.6) solid solutions up to 800 °C indicates that the molybdenum substitution leads to a remarkable improvement of the thermal stability in comparison with ZrW 2 O 8 . Based on the phase behaviors reported in this paper, an approach for the preparation of cubic Mo-rich solid solutions ZrMo 2 - x W x O 8 with attractive negative thermal expansion (NTE) properties is demonstrated.

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