Another class of materials, derived from the perovskite structure are the oxide materials known as double perovskites with the general formula AA'B2O5+ where A is a rare earth cation, A' is an alkaline earth metal cation and B is a transition metal cation. In these phases, there is cation ordering of the rare earth and alkaline earth metal layers along the (001) axis. This leads to a doubling of the c parameter when compared to the c parameter of the parent perovskite phase . The material further is an oxygen-deficient system with oxygen vacancies mainly located in the rare-earth layer. Two compositions, in particular, GdBaCo2O5+ (GBCO) and PrBaCo2O5+ (PBCO), have been studied extensively . The materials have MIEC properties but suffer from relatively lower performance and thus composites of GBCO too have been developed for increased electrochemical performance .