Dynamic structural disorder in cristobalite: neutron total scattering measurement and reverse Monte Carlo modelling

The atomic structure of cristobalite in both its high-temperature beta -pha se and low-temperature alpha -phase has been studied using constrained reve rse Monte Carlo modelling of neutron total scattering data. The modelling h as shown that the disorder of the beta -phase involves rotations and displa cements of rigid SiO4 tetrahedra, consistent with the predictions of the 'r igid-unit mode' (RUM) model. The structure of the alpha -phase is significa ntly different from that of the beta -phase, but still has a degree of diso rder. The calculated distribution of oxygen atoms in beta -cristobalite is continuous rather than based on specific crystallographic sites. There is n o evidence for correlations between the Si-O distance and the corresponding Si-O-Si bond angles, contrary to quantum mechanical calculation findings. The three-dimensional diffuse scattering has been calculated from the model configurations, and is found to be in agreement with experimental electron diffraction measurements and the predictions of the RUM model. There is li ttle significant temperature dependence of the structure or diffuse scatter ing in the beta -phase.