Acta Crystallographica Section B

Structural Science

Volume 52, Part 2 (April 1996)

research papers

Acta Cryst. (1996). B52, 223-231 [ doi:10.1107/S0108768195010524 ]

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

S. Schmid, K. Fütterer and J. G. Thompson

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 K [alpha] radiation. The composite structure consists of two component substructures referred to as the metal-substructure (M), a_M = 6.188 (1), b_M = 3.665 (1), c_M = 3.886 (1) Å, and the oxygen-substructure (O), a_O = a_M, b_O = 2.251 Å = b_M/1.628, c_O = c_M, 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(00)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 [less-than or equal to] 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 O_M along the b direction and both bond-valence sum and thermal parameter mean-square displacement along c of the metal atom.