Solution and refinement of the crystal structure of Bi7Ta3O18

The structure of heptabismuth tritantalum octadecaoxide, Bi7Ta3O18, 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 Ta5+ cations are octahedrally coordinated, with TaO6 octahedra forming columns. The remaining O atoms occupy distorted fluorite positions. The Bi3+ 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.

Pyrochlores. II. An investigation of La2Ce2O7 by neutron diffraction

The lattice parameter of La2Ce2O7 fired at 1 400 °C was found to be 5.5700 ± 7 Å (at 20 ± 2 °C). Its crystal structure was of the disordered fluorite rather than the pyrochlore type. The degree of order in the structure could not be established from the neutron powder diffraction pattern, but it is likely that the metal and oxygen atoms were distributed at random, possibly with some tendency to short-range ordering. It would appear that for Ce4+ the ionic size requirements for the formation of the pyrochlore structure are no longer satisfied, and that the cerates M23+Ce24+O7 are outside the stability limits for 3–4 pyrochlores.