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

Na3MoO3F3, a member of the A2BMVIO3F3 family of elpasolite-related oxyfluorides, has been prepared by the reaction of NaF with MoO3 at 650 deg. C. It is shown by a combined X-ray, electron and neutron diffraction study, that the true symmetry of Na3MoO3F3 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=(aR+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 Fm3-barm parent structure is consistent with a combination of phiphiphi 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 A2BMVIO3F3 family members. Bond valence arguments are used to give a plausible explanation for this difference in behavior