CRYSTAL-STRUCTURE OF LOW-TEMPERATURE FORM OF BISMUTH VANADIUM-OXIDE DETERMINED BY RIETVELD REFINEMENT OF X-RAY AND NEUTRON-DIFFRACTION DATA(ALPHA-BI4V2O11)

For the first time the structure of the polycristalline alpha - Bi4V2O 11 low-temperature form has been determined by Rietveld analysis using X-ray and neutron diffraction data in the C2/m space group. Although this phase was previously claimed to be orthorhombic, it is actually m onoclinic as we deduced from XRPD patterns. The true cell arises from a six-fold commensurate modulation of the orthorhombic mean cell along the c(M) monoclinic axis, but because of the weakness of the intensit y of some diffraction lines due to the six-fold superstructure, only t he three-fold could be investigated. alpha - Bi4V2O11 is structurally related to the parent Aurivillius prototype. However the perovskite la yers with composition (VO3.5square0.5)2- which interleave the (Bi2O2)2 + slabs are broken into files made up of alternating vanadium - oxygen tetrahedra and oxygen-deficient octahedra, running along the modulati on c(M) axis. The one-dimensional modulation depends on oxygen long-ra nge ordering and can be viewed as resulting from coupled valence modul ations of both vanadium and bismuth cations.