Compression mechanism and pressure-induced amorphization ofγ−ZrW2O8

The structure of $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{Zr}{\mathrm{W}}_{2}{\mathrm{O}}_{8}$ has been optimized at zero pressure and also at $V∕{V}_{0}=0.97$ by means of density functional theory calculations using the B3LYP functional. As previously found for $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Zr}{\mathrm{W}}_{2}{\mathrm{O}}_{8}$, tungsten polyhedra are stiffer than zirconium octahedra in $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{Zr}{\mathrm{W}}_{2}{\mathrm{O}}_{8}$. However, contrary to what has been found for $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Zr}{\mathrm{W}}_{2}{\mathrm{O}}_{8}$, all first coordination polyhedra in the $\ensuremath{\gamma}$ phase are less compressible than the unit cell. Volume reduction in $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{Zr}{\mathrm{W}}_{2}{\mathrm{O}}_{8}$ is, thus, mainly accomplished by polyhedral tilting. Upon pressure increase, the distance between the terminal oxygen and W atoms from the nearest polyhedra decreases by as much as 3.66% (for the pair O101-W6). Accordingly, a further reduction in the zirconium tungstate molar volume with the high-pressure transition to the amorphous phase should bring several oxygen atoms within the threshold of bond formation to W. $\mathrm{O}\phantom{\rule{0.2em}{0ex}}1s$ photoelectron spectra provide further experimental evidence on the formation of additional W-O bonds in amorphous zirconium tungstate. These new W-O bonds should enable the metastable retention of the amorphous phase upon pressure release.