Abstract
Although both end members in the (1−x)Ba(Li1/4Nb3/4)O3–xBa(Li2/5W3/5)O3 (BLNW) system adopt a hexagonal perovskite structure, B-site ordered cubic perovskites are formed for the majority of their solid solutions (0.238⩽x⩽0.833). Within this range, single-phase 1:2 order (P3̄m1, a≈2ac, c≈3ac) is stabilized for 0.238⩽x⩽0.385. In contrast to all known A(B1/3IB2/3II)O3 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]O3 (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)O3), 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.
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