Relationship between electrical behavior and structural characteristics in Sr-doped LaNi0.6Fe0.4O3-d mixed oxides

A series of La1-xSrxNi0.6Fe0.4O3 mixed oxides were synthesized by a coprecipitation method, followed by calcination at 1000 °C. Materials with x < 0.100 exhibited a rhombohedral structure similar to that observed for La0.4Ni0.6Fe0.4O3, whereas materials with x between 0.125 and 0.150 were orthorhombic. The unit-cell volume decreased linearly from x = 0 to x = 0.090, but increased for increasing Sr2+ content. Materials with still higher Sr content (x between 0.200 and 0.300) exhibited additional XRD reflections indicating the limit of Sr solubility into to the perovskite. The electrical conductivity of the La1-xSrxNi0.6Fe0.4O3 increased continuously up to x = 0.090, but decreased sharply when the Sr2+ content was increased to x = 0.100 or higher. However, no such discontinuity was observed in the ionic conductivities. The specific electrical behavior of these materials is discussed on the basis of equilibrium between two possible charge-compensation mechanisms. The first mechanism pictures La3+ substituted by Sr2+ in the A site of the perovskite, resulting in the oxidation of the cations hosted in the B site of the structure (Ni and/or Fe). The second charge-compensation mechanism implies the formation of one oxygen vacancy for every two Sr2+ ions.