Simulation studies of at high pressure

Abstract

The negative thermal expansion recently observed over a wide range of temperatures in the ambient pressure phase was attributed to the existence of low-frequency phonon modes which propagate with minimal distortion of the tetrahedra and 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 which also exhibited negative thermal expansion. Calculations using the rigid unit mode model have shown that the mechanism described earlier in the context of is consistent with the negative thermal expansion in . 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 . The reproduction of the cell parameters and bond lengths measured for does not imply a substantial change in the bonding character of the W-O interactions.