Non-stoichiometric 1:2 ordered perovskites in the Ba(Li1/4Nb3/4)O3-Ba(Li2/5W3/5)O3 system

Although both end members in the (1-x)Ba(Li1/4Nb3/4)O-3-xBa(Li2/5W3/5)O-3 (BLNW) system adopt a hexagonal perovskite structure, B-site ordered cubic perovskites are formed for the majority of their solid solutions (0.238less than or equal toxless than or equal to0.833). Within this range, single-phase 1:2 order (P (3) over bar m1, aapproximate toroot2a(c), capproximate toroot3a(c)) is stabilized for 0.238less than or equal toxless than or equal to0.385. In contrast to all known A((B1/3B2/3II)-B-I)O-3 perovskites, the 1:2 ordered BLNW solid solutions do not include any composition with a 1:2 cation distribution and the structure exhibits extensive non-stoichiometry. Structure refinements support a model where Li and W occupy different positions and Nb is distributed on both sites, i.e. Ba[(Li3/4+y/2Nb1/4-y/2)(1/3)(Nb1-yWy)(2/3)]O-3 (y = 0.21-0.35, where y = 0.9x). The stabilization of the non-stoichiometric order arises from the large charge/size site differences; the loss of 1:2 order for W-rich compositions is related to local charge imbalances on the A-site sub-lattice. The range of single-phase 1:1 order is confined to x = 0.833, (Ba(Li3/4Nb1/4)(1/2)(W)(1/2))O-3), where the site charge/size difference is maximized and the on-site mismatches are minimized. The microwave dielectric loss properties of the ordered BLNW solid solutions are significantly inferior as compared to their stoichiometric counterparts. (C) 2004 Elsevier Inc. All rights reserved.