Local structure in ZrW₂O₈ from neutron total scattering

The local structure of the low-temperature ordered phase of the negative thermal expansion (NTE) material ZrW₂O₈ has been investigated by reverse Monte Carlo (RMC) modelling of neutron total scattering data. The local structure is described using the instantaneous distributions of bond lengths and angles obtained from the RMC-derived models which are simultaneously consistent with the average crystal structure. These results, together with their variation with temperature, show that the majority of the mean squared displacements of the atoms are accounted for by rigid unit mode (RUM) motions of the ZrO₆ octahedra and WO₄ tetrahedra. The detailed structural description presented in this paper further supports the dominance of the basic RUM interpretation of NTE in ZrW₂O ₈ where rigid ZrO₆ and WO₄ polyhedra are joined by flexible Zr-O-W linkages.