A major barrier to widespread use of solid oxide fuel is the high operating temperature required due to the need for sufficient oxide ion diffusion through the cathode (which must also be a good electrical conductor). Improvement of the ionic conduction properties of mixed electronic-ionic conducting oxides is therefore critically important to the development of this technology. We have previously shown that the introduction of a second-row transition metal (Mo6+) on to the B-site of the cubic perovskite; (BSCF) ¿ a well known cathode material ¿ drives the formation of an endotaxial nano-composite composed of distinct single perovskite (SP) and double perovskite (DP) nano-regions, and that the resulting cathode performance is superior to that expected from the two phases in isolation due to high ionic conductivity at the interfaces here we study related systems.