Effect of La3+ doping on the perovskite-to-brownmillerite transformation in Sr1−xLaxCo0.8Fe0.2O3−δ (0≤x≤0.4)

The crystal chemistry of Sr1−xLaxCo0.8Fe0.2O3−δ (0.0≤x≤0.4) has been investigated in the temperature range 20≤T≤900 °C and oxygen partial pressure range 10−5≤pO2≤1 atm, employing X-ray diffraction, electrical conductivity, and thermogravimetric measurements. The samples adopt the perovskite structure with pseudo-cubic or cubic symmetry at room temperature. With a decreasing oxidation state of (Fe,Co) from 3.5 to 3.27+, the oxygen content increases from 2.75 to 2.84 as the La content increases from 0 to 0.4. Electrical conductivity and thermogravimetric measurements indicate a complete transformation from the cubic perovskite to the ordered orthorhombic brownmillerite phase (P/B transition) for the x=0 and 0.1 samples as the oxygen vacancy concentration δ increases. For a given temperature (T=550 °C), the equilibrium pO2 at which the P/B transition occurs shifts to lower values as the La content increases. While phase analyses after annealing under vacuum or low pO2 conditions as well as high-temperature X-ray diffraction indicate the formation of the brownmillerite phases for x≤0.3, the x=0.4 sample is found to be stable without forming the brownmillerite phase under the temperature and pO2 conditions investigated.