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

Abstract

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−δ with temperature and oxygen partial pressure variation. Neutron diffraction demonstrates that at room temperature the A-site deficient nominal composition La0.95Ni0.6Fe0.4O3−δ 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 R3̅c (167), a = 5.4491(1) Å, β = 60.761(1)°, and V = 116.372(3) Å3. NiO secondary phase was refined to rhombohedral structure with space group R3̅m (No. 166), a = 2.9560(5) Å, β = 59.99(1)°, and V = 18.260(2) Å3. High-temperature phase transformation of the perovskite phase from rhombohedral (R3̅c) to cubic (Pm3̅m) symmetry was observed above 550 °C. According to the refinement, the high temperature perovskite phase with cubic structure possesses minimal deviation from the ABO3 oxygen and cation stoichiometries. In the reduced form of the La0.95Ni0.6Fe0.4O3−δ, the cubic phase (Pm3̅m) appears above 700 °C. Several factors seem to be responsible for the increase in the temperature of the R3̅c → Pm3̅m phase transition in the reduced form of the La0.95Ni0.6Fe0.4O3−δ: 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 photoelectron spectroscopy indicates surface heterogeneity in the reduced form of La0.95Ni0.6Fe0.4O3−δ at room temperature.