A new polymorph of ZrW2O8 synthesized at high pressures and high temperatures

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 tran sition to an amorphous phase in zircon tungstate ZrW2O8 occurs above 1.5 GP a upon cold compaction. Our synchrotron angle-dispersive X-ray powder diffr action experiments show that at high temperatures the framework structure o f ZrW2O8 irreversibly collapses to a new dense hexagonal U3O8-type polymorp h at pressures just after the completion of amorphization. In this structur e, 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 pheno menon underlying the pressure-induced amorphization of zircon tungstate may involve profound changes in the coordination environments of the atoms.