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)A2+(Li1/4Nb3/4)O3-(x)A2+(Li2/5W3/5)O3 systems, A2+=Sr and Ca, within the range 0.238=<x=<0.333. The X-ray and electron diffraction patterns are consistent with a P21/c monoclinic supercell, a=6ac, b=2ac, c=32ac, β∼125 deg., where the 1:2 order is combined with b-b-c+ octahedral tilting. Rietveld refinements of the ordered A(BI1/3BII2/3)O3 structures give a good fit to a model with BI occupied by Li and Nb, BII by W and Nb, and a general stoichiometry (Sr,Ca)(Li3/4+y/2Nb1/4-y/2)1/3(Nb1-yWy)2/3O3, y=0.9x=0.21-0.30. The Sr system also includes regions of stability of a 1:3 ordered phase for 0.0=<x=<0.111, and a 1:1 ordered double perovskite for 0.833=<x=<1.0. 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