The n = 3 member of the SrCoO(3n − 1)/n series of layered oxygen-defect perovskites

Topotactic oxygen deintercalation from cubic perovskite oxide SrCoO3 − δ (δ < 0.25) is realized in a controlled way at low temperatures. Annealing in air at 200 < T < 275 °C results in a three-phase region within 0.25 < δ < 0.50. Two of the phases are known previously, namely SrCoO2.5, a phase with an orthorhombic brownmillerite-type (BM) structure, and SrCoO2.75 (T1), a phase with a slight tetragonal distortion from cubic symmetry. The third phase (T2) has a strong tetragonal distortion, and appears to be the missing n = 3 member of the SrCoO(3n − 1)/n series. Once the overall oxygen content reaches the value of (3 − δ)aver = 2.68(1), the T1-to-T2 phase conversion proceeds at constant (3 − δ)aver. A more pronounced evolution of the cT2 lattice parameter as compared to the aT2 parameter implies a layered oxygen-vacancy-ordered structure for T2 with some flexibility for oxygen-content variation, in the style of the BM phase.