Effect of Oxygen Non Stoichiometry and Oxidation State of Transition Elements on High-Temperature Phase Transition in A-Site Deficient La0.95Ni0.6Fe0.4O3-delta Perovskite

In situ high-temperature neutron powder diffraction, dilatometry, X-ray photoelectron spectroscopy (XPS), and thermal gravimetrical analysis (TGA) were applied to study crystal structure and phase transformations in A-site deficient nominal composition La0.95Ni0.6Fe0.4O3-delta with temperature and oxygen partial pressure variation. Neutron diffraction demonstrates that at room temperature the A-site deficient nominal composition La0.95Ni0.6Fe0.4O3-delta can be represented as a mixture of a stoichiometric phase with perovskite structure and NiO as a secondary phase. At room temperature the perovskite exhibits a rhombohedral structure with space group R (3) over barc (167), a = 5.4491 (1) angstrom, beta = 60 761 (1)degrees, and V = 116 372(3) angstrom(3). NiO secondary phase was refined to rhombohedral structure with space group R (3) over barm (No. 166), a = 2 9560(5) angstrom, beta = 59.99(1)degrees, and V = 18.260(2) angstrom(3) High-temperature phase transformation of the perovskite phase From rhombohedral (R (3) over barc) to Cubic (Pm (3) over barm) symmetry was observed above 550 degrees C According to the refinement. the high temperature perovskite phase with cubic structure possesses minimal deviation from the ABO(3) oxygen and cation stoichiometries. In the reduced form of the La0.95Ni0.6Fe0.4O3-delta, the cubic phase (Pm (3) over barm) appears above 700 degrees C Several factors seem to be responsible for the increase in the temperature of the R (3) over barc -> Pm (3) over barm phase transition in the reduced form of the La0.95Ni0.6Fe0.4O3-delta the increase in oxygen nonstoichiometry and the decrease in the fraction of Ni cations in high oxidation state (3+) in the perovskite constituent X-ray photoclectron spectroscopy indicates surface heterogeneity in the reduced form of La0.95Ni0.6Fe0.4O3-delta at room temperature