Phase stability of BSCF in low oxygen partial pressures

In situ X-ray diffraction has been used to investigate the phase stability of barium strontium cobalt iron oxide (BSCF) with the formula Ba0.5Sr0.5Fe1−xCoxO3−δ (x=0, 0.2, 0.4, 0.6, 0.8, and 1). The thermal decomposition processes in both low partial pressures of oxygen (air −10−5 atm pO2) and in reducing conditions have been detailed. BSCF manifests excellent stability down to 10−5 atm pO2; however, it decomposes through a complex series of oxides under reducing conditions. Increasing the cobalt content results in a decrease in the temperature range of stability of the material under 4% H2 in N2, with the initial decomposition taking place at 375, 425, 550, 600, 650 and 675 °C, for x=1, 0.8, 0.6, 0.4, 0.2 and 0, respectively. Further, the thermal expansion is a strong function of the oxygen activity and Co content. The x=0, 1 end member, BSC, undergoes a phase transition from rhombohedral to cubic symmetry at ∼800 °C under 10−5 atm pO2, resulting in an ideal perovskite with a=3.9892(3) Å at room temperature.