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 minimal distortion of the WO4 tetrahedra and ZrO6 octahedra, the so-called 'rigid unit modes'. The flexibility afforded this structure by these modes raises questions about the structure's behaviour with pressure. Further experiments found a high-pressure phase gamma-ZrW2O8 which also exhibited negative thermal expansion. Calculations using the rigid unit mode model have shown that the mechanism described earlier in the context of alpha-ZrW2O8 is consistent with the negative thermal expansion in gamma-ZrW2O8. Atomistic simulations have been used to calculate further properties of these two phases (e.g. transition pressure, compressibilities) using interatomic potentials derived for the earlier work on alpha-ZTW(2)O(8). The reproduction of the cell parameters and bond lengths measured for gamma-ZrW2O8 does not imply a substantial change in the bonding character of the W-O interactions.