Effects of niobium doping on perovskite La0.5Sr0.5Fe1−xNbxO3−δ structure

Perovskites based on lanthanum-strontium ferrite are considered promising electrode materials for use in various types of fuel cells. The strategy of modifying these materials by partial substitution of iron with highly charged ferroactive cations has proven to be an effective way to enhance their chemical stability. It was shown here that La0.5Sr0.5Fe1−xNbxO3−δ forms a continuous series of substitutional solid solutions up to niobium concentration x = 0.1. It was found for the first time that a further increase in the niobium concentration leads to a transformation of initial rhombohedral (hexagonal) perovskite structure (R3‾c) to less symmetrical structure of orthorhombic perovskite (Pbnm). The latter also forms a number of substitutional solid solutions in the range 0.1 < x ≤ 0.25. High-temperature diffraction analyses of perovskites of composition La0.5Sr0.5Fe1−xNbxO3−δ (x = 0.03, 0.05, 0.1) were carried out in the air atmosphere and in vacuum. It was demonstrated that with an increase in the Nb content in the perovskite, the temperature of the "hexagonal-cubic" phase transition goes up, which indicates an increase in the structural stability of the oxide in the range of operating temperatures of SOFCs/SOECs.