SIMULATION STUDIES OF ZRW2O8 AT HIGH-PRESSURE

The negative thermal expansion recently observed over a wide range of temperatures in the ambient pressure phase alpha-ZrW2O8 was attributed to the existence of low-frequency phonon modes which propagate with m inimal distortion of the WO4 tetrahedra and ZrO6 octahedra, the so-cal led 'rigid unit modes'. The flexibility afforded this structure by the se modes raises questions about the structure's behaviour with pressur e. Further experiments found a high-pressure phase gamma-ZrW2O8 which also exhibited negative thermal expansion. Calculations using the rigi d unit mode model have shown that the mechanism described earlier in t he context of alpha-ZrW2O8 is consistent with the negative thermal exp ansion in gamma-ZrW2O8. Atomistic simulations have been used to calcul ate further properties of these two phases (e.g. transition pressure, compressibilities) using interatomic potentials derived for the earlie r work on alpha-ZTW(2)O(8). The reproduction of the cell parameters an d bond lengths measured for gamma-ZrW2O8 does not imply a substantial change in the bonding character of the W-O interactions.