STRUCTURES OF TA22W4O67 AND TA74W6O203 .1. REFINED STRUCTURAL MODELS USING SYNCHROTRON-RADIATION

The crystal structures of Ta22W4O67 [M(r) = 5788.19, a = 6.1485 (5), b = 47.6205 (12), c = 3.8559 (3) Angstrom, gamma = 90.04 (1)degrees, sp ace group = C112/m (non-standard setting), Z = 1, D-x = 8.513 g cm(-3) , F(000) = 2438] and Ta74W6O203 [M(r) = 17741.06, a = 6.1664 (5), b = 29.2717 (14), c = 3.8731 (2) Angstrom, space group = Pbam (no. 55), Z = 0.2, D-x = 8.428 g cm(-3) F(000) = 1494] were determined using synch rotron radiation at four different wavelengths below the TaL(III) edge [lambda = 1.2741 (-146eV), lambda = 1.2586 (-26eV), lambda = 1.2571 ( -14eV) and lambda = 1.2563 Angstrom (-8 eV)]. The collection of data i mmediately below the TaL(III), edge at -8eV enabled resolution of Ta a nd W of up to eight electrons, which assisted in the refinement of Ta/ W ordering for both structures. Bond valence arguments have been used to locate oxygen vacancies required by the formulae. From the largest data set for Ta22W4O67 (lambda = 1.2741 Angstrom, a final value of 0.0 481 for R(1) = Sigma \\F-obs(h)\ - \F-calc(h)\\/Sigma\F-obs(h)\ was ob tained for 3082 unmerged reflections with I(h) > 3 sigma[I(h)] and for Ta74W6O203 (lambda = 1.2563 Angstrom) a final value of 0.0571 for R(1 ), was obtained for 5675 unmerged reflections. The two structures are described from a conventional polyhedral perspective as 13- and 8-time s superstructures occurring in the solid solution (1-x)Ta2O5-xWO(3), 0 less than or equal to x less than or equal to 0.267.