Structural analysis of several W(VI) and Mo(VI) complex perovskites prepared by the polymeric precursors method

We describe in this work the synthesis by the Pechini method of five Mo(VI)- and W(VI)-containing complex perovskites and their structural characterisation by HREM and XRD. The compounds studied, Ba(B'B-2/3"(1/3))O-3 (B'= In and Y; B" = W and Mo) and Sr(In2/3W1/3)O-3, were obtained after firing the precursor powders for 8 h at 1200degreesC. Thermal analysis showed that the formation mechanism of the five perovskites is similar and implies the formation of barium carbonate and barium tungstates or molybdates of different stoichiometries as intermediate phases. Interesting enough, these similar mechanisms yield to materials of a quite different structure. Indeed, Ba(In2/3MO1/3)O-3 and Sr(In2/3W1/3)O-3 were found to be disordered perovskites (unit cells: a(p) x a(p) x a(p) and root2a(p) x root2a(p) x 2a(p), respectively); on the other hand, two phases coexist in the sample Ba(In2/3W1/3)O-3 at the synthesis conditions: an ordered predominant phase (unit cell: 2ap x 2ap x 2ap) and, as a minor phase, a disordered perovskite (unit cell: ap x ap x ap). Finally, the two yttrium-containing compounds were found to be ordered perovskites (2a(p) x 2a(p) x 2a(p)). (C) 2003 Elsevier Science (USA). All rights reserved.