Effect of annealing on the elastic moduli of amorphous zirconium tungstate

Amorphous zirconium tungstate (a-ZrW2O8), prepared at 7.7 GPa, was subjected to successive heat treatments at increasingly higher temperatures, up to 640 °C. Evidence of recrystallization into α-ZrW2O8 was found in the room temperature X-ray powder diffraction pattern of samples previously heated above 600 °C. Changes in elastic moduli and linear thermal expansion coefficient upon annealing of a-ZrW2O8 up to 600 °C cannot be accounted for by assuming a composite sample consisting of α-ZrW2O8 domains dispersed into an amorphous zirconium tungstate matrix. The irreversible relaxation of a-ZrW2O8 and recrystallization into α-ZrW2O8 is accompanied by an increase of bulk modulus (from 61.8(8) GPa to 74.4(9) GPa) and Poisson's ratio (from 0.267(3) to 0.302(3)) and diminution of density (from 6.54(1) g/cm3 for a-ZrW2O8 to 5.18(1) g/cm3 for α-ZrW2O8). Despite the greater density, a-ZrW2O8 (as well as γ-ZrW2O8, a high pressure polymorph) is more compressible than α-ZrW2O8, possibly owing to the new degrees of freedom provided by lowering symmetry, which allow external stresses to be accommodated by polyhedral tilting. Furthermore, the observed increase of Poisson's ratio upon thermal treatments at increasingly higher temperatures indicates that annealing of a-ZrW2O8 is accompanied by the irreversible reduction in the average number of bridging oxygen atoms.