Solution and refinement of the crystal structure of Bi7Ta3O18

The structure of heptabismuth tritantalum octadecaoxide, Bi(7)Ta(3)O(18), has been solved and refined using single-crystal X-ray diffraction data collected at a synchrotron source in conjunction with unit-cell and symmetry information derived from electron diffraction. The space-group symmetry is triclinic 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 prototype fitted all the Rietveld-refinable features of a medium-resolution neutron 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 unit cells. The metal-atom populations and O-atom positions are fully ordered. The Ta(5+) cations are octahedrally coordinated, with TaO(6) octahedra forming columns. The remaining O atoms occupy distorted fluorite positions. The Bi(3+) cations occupy octahedral, square pyramidal or trigonal prismatic sites within the O-atom array; strain in the latter coordination environment appears to be responsible for the lowering of symmetry from monoclinic to triclinic.