CATION ORDERING TRANSFORMATIONS IN BA(MG1 3TA2/3)O-3-BAZRO3 PEROVSKITE SOLID-SOLUTIONS/

The effect of the substitution of BaZrO3 on the cation ordering in Ba( Mg1/3Ta2/3)O-3 was studied using TEM and X-ray and neutron diffraction . Almost no solubility of Zr was found in the 1:2 ordered, trigonal st ructure of the Ba(Mg1/3Ta2/3)O-3 end-member (P (3) over bar m 1), and a transformation to a 1:1 ordered, cubic (Fm (3) over bar m) phase wit h a = 2a(per) occurs for substitution levels between similar to 10 and 25 mol% BaZrO3. The structure of this Ba(beta'(1/2)beta ''(1/2))O-3-t ype phase consists of two distinct octahedral sites, beta' and beta '' . The occupancies of the two cation positions, refined using the Rietv eld method, were found to be consistent with a ''random site'' model i n which beta '' is occupied by Ta, and beta' by a random distribution of the remaining cations. The homogeneity range of the 1:1 solid solut ions predicted by this model, Ba{[Mg(2-y/3) Ta(2-2y/3) ZT(y)](1/2)[Ta] (1/2)}O-3 (0.0 < y less than or equal to 0.5), is in excellent agreeme nt with that observed experimentally. (C) 1997 Elsevier Science Ltd.