Dynamic structural disorder incristobalite: neutron total scattering measurement and reverseMonte Carlo modelling

Abstract

The atomic structure of cristobalite in both its high-temperatureβ-phase and low-temperature α-phase has been studied usingconstrained reverse Monte Carlo modelling of neutron totalscattering data. The modelling has shown that the disorder of theβ-phase involves rotations and displacements of rigid SiO4tetrahedra, consistent with the predictions of the `rigid-unit mode'(RUM) model. The structure of the α-phase is significantlydifferent from that of the β-phase, but still has a degree ofdisorder. The calculated distribution of oxygen atoms inβ-cristobalite is continuous rather than based on specificcrystallographic sites. There is no evidence for correlationsbetween the Si-O distance and the corresponding Si-O-Si bondangles, contrary to quantum mechanical calculation findings. Thethree-dimensional diffuse scattering has been calculated from themodel configurations, and is found to be in agreement withexperimental electron diffraction measurements and the predictionsof the RUM model. There is little significant temperaturedependence of the structure or diffuse scattering in the β-phase.