Nuclear magnetic resonance analysis and activation energy spectrum of the irreversible structural relaxation of amorphous zirconium tungstate

Zirconium tungstate undergoes a sequence of phase transitions from cubic (α−ZrW2O8) to orthorhombic (γ−ZrW2O8) to amorphous (a−ZrW2O8) upon increasing pressure at room temperature. The amorphous phase is known to undergo anomalous endothermic recrystallization into a high-temperature β−ZrW2O8 phase above 600∘C at ambient pressure (and back to α−ZrW2O8 when brought to room temperature). The endothermic recrystallization of a−ZrW2O8 is preceded by an irreversible exothermic structural relaxation. New W-O bonds are formed upon amorphization, continuing a tendency of increasing W coordination number in going from α to γ−ZrW2O8. In fact, contrarily to α−ZrW2O8, in which one-quarter of the oxygen atoms are bonded only to one W (terminal oxygens), previous works found no evidence of single-bonded oxygen atoms in a−ZrW2O8. It thus could be argued that the irreversible character of the structural relaxation of a−ZrW2O8 is due to W-O bond breaking upon annealing of the amorphous phase. To test this hypothesis, x-ray diffraction, O17 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 2.5eV. These findings suggest that the structural relaxation of amorphous zirconium tungstate, however irreversible, is not accompanied by W-O bond breaking, but most probably characterized by a succession of (mostly) irreversible local atomic rearrangements.