THE INCOMMENSURATELY MODULATED (1-X)TA2O5-BULLET XWO3, 0 LESS-THAN-OR-EQUAL-TO X LESS-THAN-OR-EQUAL-TO 0.267 SOLID-SOLUTION

The phase(1 - x)Ta2O5 · WO3, 0 ≤ x ≤ 0.267 has been studied by X-ray powder diffraction and transmission electron microscopy. It was previously described as an infinite series of anion-deficient, α-UO3-type "line phases," with compositions resulting from intergrowths of different blocks made up by small numbers of α-UO3-type cells. More correctly(1 - x)Ta2O5· xWO3, 0 ≤ x ≤ 0.267 is described as an incommensurately modulated structure with a linearly composition-dependent primary modulation wave-vector qprim. =qb*. The underlying orthorhombically distorted α-UO3-type parent structure has space group symmetry Cmmm (a ≈ 6.20-6.14, b ≈ 3.66, c ≈ 3.89-3.85Å). Characteristic extinction conditions imply a superspace group symmetry of P : Cmmmm : s, -1,1. The four previously reported crystal structures in the solid solution field are examined by means of apparent valence calculations. Crystal chemical reasons are proposed for the width of the composition range,0 ≤ x ≤ 0.267, observed for the title phase. © 1992 Academic Press, Inc.