The electrochemical and chemical oxidation of the defect perovskite SrCoO 2.5 to the cubic perovskite SrCoO 3 at ambient temperature in alkaline electrolyte are shown to be reversible processes with the appearance of intermediate compounds; the phase diagram can be described by two line phases SrCoO 2.5 and SrCoO 2.75 and one solid solution range SrCoO 2.87+y with 0 < y < 0.13. The redox cycle displays a structural hysteresis under dynamic conditions. Several models are discussed in order to explain the unusual phenomenon of roomtemperature bulk oxygen transport.

The electrochemical and chemical oxidation of the defect perovskite SrFeO 2.5 to the cubic perovskite SrFeO 3 at ambient temperature in alkaline electrolyte are shown to be in principle reversible processes with the appearance of intermediate compounds; the oxidation can be described by the formation of line phases SrFeO 2.75 and SrFeO 2.87 and one solid solution range SrFeO 3-x with 0 e x e 0.03. The structure of the brownmillerite type parent phase is characterized by the presence of stacking faults in the octahedral and tetrahedral sequence of layers whose concentration increases significantly upon formation of the intermediate phases. Models based on observed microscopic features are discussed in order to explain the unusual phenomenon of room temperature oxygen transport.

A new approach to the determination of oxygen nonstoichiometry (δ) of MIEC oxides as a continuous function of pO2 at high temperatures was developed. The description of the model allowing one to distinguish the contribution of oxygen released from the samples to the partial pressure of oxygen at the outlet of the continuous-flow fixed-bed reactor after the stepwise change of the oxygen partial pressure of inlet gas from 0.2 to 10(-5) atm and to calculate the dependence of δ on pO2 is presented. The criterion for assessing the achievement of quasi equilibrium release of oxygen from the MIEC oxides is proposed. The adequacy of the method was confirmed by comparing the obtained and published data for well-studied SrCo0.8Fe0.2O3-δ and SrFeO3-δ MIEC oxides.

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