ORDER-DISORDER TRANSITION OF BAM2O4 BODIE S (M LA, ND, SM, GD, HO OR Y) SYNTHESIZED BY SINTERING OF BACO3-M2O3 MIXTURES

Sintered bodies of BaM2O4 (M: La, Nd, Sm, Gd, Ho or Y) with CaFe2O4-re lated structure were synthesized by sintering BaCO3-M2O3 Mixtures. The temperature dependence of oxygen ionic conductivity and the relations hip between the ionic radius of M3+ ions in B sites and the crystal st ructure of BaM2O4 were investigated. The ionic conductivity type in Ba M2O4 was divided into two groups; (a) BaLa2O4, BaNd2O4 and BaSM2O4 wit h order-disorder transition and (b) BaGd2O4, BaHo2O4 and BaY2O4 withou t the transition. There were vacant spaces in the crystal structure of BaNd2O4 and BaSM2O4 with order-disorder transition due to the distort ion in the a-axis direction of an oxygen octahedron. By assuming that the vacant space is an oxygen vacancy, the ionic conductivity in BaLa2 O4, BaNd2O4 and BaSM2O4 has been explained with a model of the order-d isorder transition for oxygen vacancies without significant shifting o f cations in the crystal lattice.