Phase relations of BaCoO3 − δ–BaInO2.5 and size variation effect of B-site cations on the phase transitions

The phase relations in the system of BaCoO3 − δ–BaInO2.5 in the temperature range 800–1150 °C were investigated and five phases with perovskite or its related structures were identified. In the range of 1050 to 1150 °C, there exists a wide solid solution BaInxCo1 − xOy (0.1 ≤ x ≤ 0.84, 2.54 ≤ y ≤ 2.59) with the cubic perovskite structure. At 1050 °C, the cubic phase transforms to an orthorhombic phase firstly at x = 0.55 and the cubic solid solution is separated into two regions, and narrowed with the decrease of the temperature. The structure of the cubic phase in In-rich region appears as a super-lattice (a′) of the simple cubic phase (ap) with the modulation vector a′ = 2ap. At 950 °C, a tetragonal phase BaIn0.64Co0.36Oy emerges and extends to a solid solution at lower temperature. The orthorhombic and tetragonal structures are studied by electron diffraction and powder X-ray Rietveld analysis. The size variance of B-site cations was applied to interpret the phase transition from the cubic to the orthorhombic at x = 0.55 for compounds BaInxCo1 − xOy. The cubic solid solution shows good conductivities at low temperature.