A New Polymorph of ZrW₂O₈ Synthesized at High Pressures and High Temperatures

Abstract

Pressure-induced amorphization of materials with negative thermal expansion is believed to arise from uncorrelated tiltings of polyhedra, kinetically frozen in an orientationally disordered state at room temperature. The transition to an amorphous phase in zircon tungstate ZrW2O8 occurs above 1.5 GPa upon cold compaction. Our synchrotron angle-dispersive X-ray powder diffraction experiments show that at high temperatures the framework structure of ZrW2O8 irreversibly collapses to a new dense hexagonal U3O8-type polymorph at pressures just after the completion of amorphization. In this structure, the Zr and W atoms are 6-fold coordinated and statistically disordered. Upon heating at higher pressures, the amorphous ZrW2O8 material decomposes into its constituent oxides. The results of our work suggest that the phenomenon underlying the pressure-induced amorphization of zircon tungstate may involve profound changes in the coordination environments of the atoms.