Relationship between Electrical Behavior and Structural Characteristics in Sr-Doped LaNi_0.6Fe_0.4O_3−δ Mixed Oxides

A series of La 1− x Sr x Ni 0.6 Fe 0.4 O 3 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 La 0.4 Ni 0.6 Fe 0.4 O 3, 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 Sr 2+ 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 La 1− x Sr x Ni 0.6 Fe 0.4 O 3 increased continuously up to x = 0.090, but decreased sharply when the Sr 2+ 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 La 3+ substituted by Sr 2+ 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 Sr 2+ ions.