A combined diffraction (XRD, electron and neutron) and electrical study of Na₃MoO₃F₃

Na₃MoO₃F₃, a member of the A ₂BM^VIO₃F₃ family of elpasolite-related oxyfluorides, has been prepared by the reaction of NaF with MoO₃ at 650°C. It is shown by a combined X-ray, electron and neutron diffraction study, that the true symmetry of Na₃MoO ₃F₃ is not monoclinic (pseudo-orthorhombic) as previously reported but instead triclinic (metrically rhombohedral) P1. The superstructure unit cell is given by a=-a_R+b_R, b=c_R, c=(a _R+b_R+c_R), when expressed with respect to the underlying rhombohedral parent structure. Neutron diffraction refinement of the rhombohedral sub-structure shows O and F to be fully ordered with alternate (111)_R planes being occupied by O and then F. The nature of the distortion away from the high temperature Fm3̄m parent structure is consistent with a combination of φφφ type rotations of the octahedral framework together with ppp type displacements of the octahedral cations, which is not observed in any of the closely related A ₂BM^VIO₃F₃ family members. Bond valence arguments are used to give a plausible explanation for this difference in behavior.