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.
Ling
CD
Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia. email: ling@rsc.anu.edu.au
Schmid
S
Withers
RL
Thompson
JG
Ishizawa
N
Kishimoto
S