STRUCTURES OF TA22W4O67 AND TA74W6O203 .2. A COMPOSITE MODULATED STRUCTURE APPROACH TO THE (1-X)TA(2)O(5)XWO(3), SS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-0.267, SOLID-SOLUTION

The room-temperature commensurate single-crystal structures of Ta22W4O 67 [M(r) = 5788.19, a = 6.1485(5) b = 47.6205(12), c = 3.8559 (3)Angst rom, gamma = 90.04(1)degrees C112/m, Z = 1, D-x = 8.513 g cm(-3)] and Ta74W6O203 [M(r) = 17741.06, a = 6.1664 (5), b = 29.2717(14), c = 3.87 31(2)Angstrom, Pbam, Z = 0.2, D-x = 8.428 g cm(-3)] have each been ref ined using data sets collected with synchrotron radiation at three wav elengths near the Ta L(III) absorption edge. The structures are parame terized and refined as composite modulated structures, composed of two interacting Cmmm parent substructures whose relative origins are sepa rated by 1/4a + delta b. The parent substructures have the same a and c axial lengths, but the b axial lengths mismatch. Characteristic exti nction conditions when the substructures are mutually incommensurable imply a superspace-group symmetry of C'mmm(0,alpha,0)s,-1,1 with alpha similar to 1.62. The refined atomic modulation functions describing t he structural deviation of both substructures from their underlying pa rent substructures are shown to be remarkably similar for these two qu ite different compositions and are, therefore, most probably represent ative of the whole solid-solution range.