Thermal expansion in the orthorhombic gamma phase of ZrW2O8

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