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

Abstract

Na3MoO3F3, a member of the A 2BMVIO3F3 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=-aR+bR, b=cR, c=(a R+bR+cR), 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 2BMVIO3F3 family members. Bond valence arguments are used to give a plausible explanation for this difference in behavior.