Solution and refinement of the crystal structure of Bi7Ta3O18

The structure of heptabismuth tritantalum octadecaoxide, Bi7Ta3O18, has bee n solved and refined using single-crystal X-ray diffraction data collected at a synchrotron source in conjunction with unit-cell and symmetry informat ion derived from electron diffraction. The space-group symmetry is triclini c C1 but is very close to monoclinic C2/m. A twin component observed during data collection was successfully modelled in the refinement. The C2/m prot otype fitted all the Rietveld-refinable features of a medium-resolution neu tron powder diffraction pattern. The metal-atom array is approximately face -centred cubic (fluorite type), punctuated by regularly spaced displacement faults perpendicular to the [111](fluorite) direction every 2.5 fluorite u nit cells. The metal-atom populations and O-atom positions are fully ordere d. The Ta5+ cations are octahedrally coordinated, with TaO6 octahedra formi ng columns. The remaining O atoms occupy distorted fluorite positions. The Bi3+ cations occupy octahedral, square pyramidal or trigonal prismatic site s within the O-atom array; strain in the latter coordination environment ap pears to be responsible for the lowering of symmetry from monoclinic to tri clinic.