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Strontium cobaltates in the composition range, SrCoO2.29‐SrCoO2.80, have been prepared under various conditions with respect to the oxygen pressure and temperature. Samples prepared at high temperatures (above 800°C) crystallized in the brownmillerite or perovskite type Structure Depending on oxygen Deficiency. A perovskite phase having large oxygen Deficiency, SrCoO2.29, and a brownmillerite solid solution, SrCoOx (2.42 < x < 2.52), were observed. On the other hand, samples annealed at low temperatures (below 800°C) did not crystallize in 2H type Structure with Sr:Co ratio of 1:1 but in the cobalt Deficient phase, 2H‐SrCo1‐uOx (u ≈︁ 0.1) and Co3O4. The high oxygen content perovskite SrCoO2.67‐2.80 was obtained by annealing the brown‐millerite phase under high oxygen pressures at 300°C. The electrical conductivity of SrCoOx (2.29 < x < 2.80) and the oxygen diffusion coefficient of the perovskite phase SrCoOx (x = 2.68, 2.75 and 2.80) were measured as a function of temperature. SrCoO2.80 showed the very high diffusion coefficient of 2.3 × 10−11 cm2 sec−1 at 23°C.

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