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The structure of ␥-ZrW 2 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 ␣-ZrW 2 O 8 , tungsten polyhedra are stiffer than zirconium octahedra in ␥-ZrW 2 O 8 . However, contrary to what has been found for ␣-ZrW 2 O 8 , all first coordination polyhedra in the ␥ phase are less compressible than the unit cell. Volume reduction in ␥-ZrW 2 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-W6͒. 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. O 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.

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