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 (, , ) 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.

A new 1:2 ordered perovskite La(Li1/3Ti2/3)O3 has been synthesized via solid-state techniques. At temperature >1185°C, Li and Ti are randomly distributed on the B-sites and the X-ray powder patterns can be indexed in a tilted (b−b−c+) Pbnm orthorhombic cell (a=ac√2=5.545 Å, b=ac√2=5.561 Å, c=2ac=7.835 Å). However, for T⩽1175°C, a 1:2 layered ordering of Li and Ti along 〈111〉c yields a structure with a P21/c monoclinic cell with a=ac√6=9.604 Å, b=ac√2=5.552 Å, c=ac3√2=16.661 Å, β=125.12°. While this type of order is well known in the A2+(B2+1/3B5+2/3)O3 family of niobates and tantalates, La(Li1/3Ti2/3)O3 is the first example of a titanate perovskite with a 1:2 ordering of cations on the B-sites.

High temperature thermal treatments were used to modify the cation order in several tantalate and niobate members of the Pb(Mg1/3Nb2/3)O3 (PMN) family of relaxors. The observation of complete 1:1 structural order in several compositions, and the refined cation occupancies of well-ordered samples conflict with the predictions of the “space charge” model, and support the “random site” description for the B-site order. In this charge balanced model one of the positions in the ordered structure is solely occupied by Ta (Nb), while the other contains a random distribution of Mg and the remaining Ta (Nb) cations. The stability of the order and magnitude of the domain growth is strongly influenced by solid solution additives. For Pb(Mg1/3Ta2/3)O3 (PMT), ordering-enhancing Zr, Sc, or La substituents increase the cation order–disorder transition temperature (∼1375°C in pure PMT) and promote extensive domain coarsening. For pure PMN a low temperature (<1000°C) order–disorder transition prevents any structure modification, and domain growth could only be realized with additives (Tb, Sc, or La) that stabilize the order to temperatures where the samples are “kinetically active”. The retention of relaxor behavior in all the fully 1:1 ordered, large-domain PMT and PMN-based ceramics suggests that the disorder on the random site is critical in frustrating ferroelectric order. By systematically controlling the concentration of ferroelectrically active cations on this position in fully ordered (1−x)Pb(Mg1/3Ta2/3)O3–(x)Pb(Sc1/2Ta1/2)O3 solid solutions, a crossover from relaxor to normal ferroelectric behavior was induced at x=0.5.