The structure of the oxygen-deficient compound 7Bi₂O₃^.2WO₃, a fluorite-derivative phase considered a candidate for electronic applications because of its high ionic conductivity, is investigated by single-crystal X-Ray diffraction employing Ag-K α radiation (lambda = 0.5608 Å) to minimize the effect of the absorption by the heavy metals. The space-group type is I4₁, the acentric subgroup of I4₁/a that was previously suggested from powder-diffraction data and precession-camera photographs. Lattice parameters are a = 12.513(2), c = 11.231(4) (Z = 2.5). The sample is twinned by syngonic merohedry, class I, with volume ratio of the individuals 0.58/0.42. The ordering of W partly confirms previous models, with one W fully occupying one of the sites on special position. However, the remaining W goes in a site on general position, which shares with Bi, resulting thus more diluted in the structure. The oxygen vacancies are partly ordered in three of the ten anion sites.

Keywords: bismuth-tungsten oxide, ion-conductor, single-crystal X-ray diffraction, twinning

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