At the measurement temperature of 800 ˚C, oxygen vacancies are the lowest energy defects. The oxygen vacancy formation energy in Fig. increases layer by layer from the centre of the perovskite layer O(1) to the apical oxygen within the rocksalt layer (O4) in the sequence: ) around the NiO6 octahedra is expected and in agreement with the published literature for materials such as La0.3Sr2.7CoFeO7-, LaSr3Co1.5Fe1.5O10-, LaSrCo0.5Fe0.5O4- and La1-xSrxBO3 (B = Cr, Mn, Fe and Co.) .