Zirconium tungstate (ZrW₂O₈ ) is a ceramic material which exhibits several unusual behaviours, including negative thermal expansion, pressure-induced amorphization (between 1.5 GPa and 2.0 GPa) and anomalous endothermic recrystallization when the amorphous phase (a-ZrW₂O₈) is heated to temperatures above 600°C at room pressure. An irreversible exothermic structural relaxation precedes the endothermic recrystallization of a-ZrW₂O₈. This relaxation is characterized by a continuous activation energy spectrum. In this work we explore the mechanism of structural relaxation of the amorphous phase of zirconium tungstate, aiming to determine if this phenomenon involves the breaking of the W-O bonds formed during amorphization. For this purpose, x-ray diffraction, ₁₇O magic-angle spinning NMR, Raman, and far-infrared analyses were performed on samples of amorphous zirconium tungstate previously annealed to increasingly higher temperatures, looking for any evidence of features that could be assigned to the presence of terminal oxygen atoms. No evidence of single-bonded oxygen was found before the onset of recrystallization. Furthermore, the kinetics of the structural relaxation of a−ZrW₂O₈ is consistent with a continuous spectrum of activation energy, spanning all the range from 1 to 2.5 eV. These findings suggest that the structural relaxation of amorphous zirconium tungstate is irreversible but is not accompanied by W-O bond breaking. It is probably characterized by a succession of (mostly) irreversible local atomic rearrangements.

The support from the Brazilian agencies CNPq - grants 304831/2014-0 (CAP) and 304675/2015-6 (JEZ), PRONEX/FAPERGS, and CAPES - Finance Code 001 is gratefully acknowledged. Thanks are also due to Netzsch Ger¨atebau GmbH (Germany), Jim Walker (Wah Chang Co., Albany, OR), for the sample of zirconium tungstate used in this work, Aline Lima de Oliveira (Universidade de Bras´ılia, Brazil), who carried out the NMR measurements reported in this work.