Ordered perovskites in the A2+ (Li1/4Nb3/4)O3-A2+ (Li2/5W3/5)O3 (A2+ = Sr, Ca) systems

Single-phase 1:2 B-site ordered perovskites are formed in the (1-x)A(2+) (Li1/4Nb3/4)O-3-(x)A(2+) (Li1/5W3/5)O-3 systems, A(2+) = Sr and Ca, within the range 0.238less than or equal toxless than or equal to0.333. The X-ray and electron diffraction patterns are consistent with a P2(1)/c monoclinic supercell, a = root6a(c), b = root2a(c), c = 3root2a(c), beta approximate to 125degrees, where the 1:2 order is combined with b(-)b(-)c(+) octahedral tilting. Rietveld refinements of the ordered A((B1/3B2/3II)-B-I)O-3 structures give a good fit to a model with B-I occupied by Li and Nb, B-II by W and Nb, and a general stoichiometry (Sr,Ca)(Li3/4 + y/2 Nb1/4-y2)(1/3)(Nb1-yWy)(2/3)O-3, y = 0.9x = 0.21 - 0.30. The Sr system also includes regions of stability of a 1:3 ordered phase for 0.0less than or equal toxless than or equal to0.111, and a 1:1 ordered double perovskite for 0.833less than or equal toxless than or equal to1.0. The formation of the non-stoichiometric 1:2 ordered phases is associated with the large site charge/size differences that can be accessed in these systems, and restricted by local charge imbalances at the A-sites for W-rich compositions. These concepts are used to generate stability maps to rationalize the formation of the known 1:2 ordered oxide perovskites. (C) 2004 Elsevier Inc. All rights reserved.