X-ray structure refinement of (3 + 1)-dimensional incommensurate composite (1-x)Ta2O5.xWO3, x = 0.1

Abstract

The structure refinement of tantalum tungstate, (1−x) Ta2O5.xWO3, x = 0.1, as a (3 + 1)-dimensional incommensurately modulated composite structure was carried out using X-ray single-crystal diffraction data measured with Mo Kα radiation. The composite structure consists of two component substructures referred to as the metal-substructure (M), aM = 6.188 (1), bM = 3.665 (1), cM = 3.886 (1) Å, and the oxygen-substructure (O), aO = aM , bO = 2.251 Å = bM /1.628, cO = cM , respectively. The primary modulation wavevector of the M substructure is chosen to be q M = b O * = 1.628 b M * . The overall superspace-group symmetry is C′mmm(0β0)s00. The structure was refined to an overall wR = 0.0561. The structures of the compounds x = 0.267 (Ta22W4O67) and x = 0.14 (Ta74W6O203), also members of the (1 − x) Ta2O5.xWO3, 0 ≤ x ≤ 0.267, solid solution, were rerefined as incommensurately modulated structures using intensity data reported previously. The displacive modulation wave amplitudes obtained from these refinements are in remarkable agreement with each other. Beyond the similarity of the atomic modulation functions across the solid solution, the structures show subtle but significant differences as a function of composition in the following ways: displacement of atom OM along the b direction and both bond-valence sum and thermal parameter mean-square displacement along c of the metal atom.