Thermal expansion in the orthorhombic γ phase of ZrW2O8

The thermal expansion of the orthorhombic gamma phase of ZrW2O8 has been measured using neutron powder diffraction from 4.6 to 410 K, where it transforms to the cubic ct phase. At low temperature, gamma-ZrW2O8 has a negative thermal expansion, but the thermal expansion becomes:less negative with increasing temperature and is slightly positive at room temperature. This behavior can be! explained in terms of the contributing phonon modes: At low temperature, the vibrational modes lead to a negative:thermal expansion, but additional modes that become active upon increasing temperature add positive contributions. Above room temperature, the a and b axes increase more sharply while the c axis reverses its behavior and decreases with increasing temperature. This unusual behavior can be explained in terms of a thermally activated process, presumed to result from oxygen-atom migration, that makes an additional contribution to the thermal expansion. [S0163-1829(99)03346-9].