EFFECT OF M4-SITE SUBSTITUTIONS ON THE CATION ORDERING IN BA(MG1( (CE, SN, TI) B)3TA2/3)O-3/

The effect of different tetravalent octahedral site substitutions on t he cation ordering in Ba(Mg1/3Ta2/3)O-3 perovskites were explored by i nvestigating the phase stabilities and structures of their solid solut ions with BaTiO3, BaSnO3, and BaCeO3. In the cerate system, the 1:2 or dered form of Ba(Mg1/3Ta2/3)O-3 has very limited stability and transfo rms to a cubic, 1:1 ordered structure with a doubled perovskite repeat between 7 and 25 mol% BaCeO3. Structure refinements of the Ba(beta'(1 /2)beta ''(1/2))O-3 phases indicate that the 1:1 order can be describe d by a ''random site'' model. In this model, the beta '' sites are occ upied by Ta, the beta' sites are occupied by a random distribution of Mg, Ce, and the remaining Ta cations, and the stoichiometry of the str ucture can be represented by Ba[Mg2(1-x/3)Ta(1-4x/3)Ce-2x](1/2)[Ta](1/ 2)O-3. Although the formation of this phase in the cerate system is si milar to the results reported previously for BaZrO3 substitutions, no evidence of 1:1 order was found in the BaTiO3 solid solutions, which a dopt a disordered perovskite structure for >10 mol% Ti. The correlatio n between the size of the tetravalent cation and the formation of the- random site structure extended to the BaSnO3 system, which also showed no evidence of an extended region of stable 1:1 order. (C) 1998 Elsev ier Science Ltd.