Oxygen/fluorine ordering, structured diffuse scattering and the local crystal chemistry of K₃MoO₃F₃

Bond valence sum calculations are used to investigate the crystal chemistry of the elpasolite-related oxyfluoride K₃MoO ₃F₃ in order to obtain insight into the type/s of structural distortion (away from an ideal, high symmetry, elpasolite type parent structure) responsible for a characteristic, highly structured, three-dimensional diffuse intensity distribution. The first required type of local structural distortion corresponds to large amplitude MoO₃F ₃ octahedral rotations while the second is associated with O/F ordering and associated induced Mo ion shifts. Monte Carlo modelling is used to show how the latter when coupled with an appropriate local crystal chemical constraint can give rise to the observed structured diffuse scattering. The study is part of a wider search for diffraction evidence of oxygen/fluorine ordering in metal oxyfluoride systems.