Effect of oxygen non stoichiometry and oxidation state of transition elements on high-temperature phase transition in a-site deficient La _0.95Ni_0.6Fe_0.4O_3-δ 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 La_0.95Ni_0.6Fe_0.4O _3-δ with temperature and oxygen partial pressure variation. Neutron diffraction demonstrates that at room temperature the A-site deficient nominal composition La_0.95Ni_0.6Fe_0.4O _3-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 R3̄c (167), a= 5.4491(1) Å, β = 60.761(1)°, and V = 116.372(3) ų. NiO secondary phase was refined to rhombohedral structure with space group R3̄m (No. 166), a=2.9560(5) Å, β=59.99(1)°, and V=18.260(2) ų. High-temperature phase transformation of the perovskite phase from rhombohedral (R3̄c) to cubic (Pm3̄m) 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 La_0.95Ni_0.6Fe_0.4O_3-δ, the cubic phase (Pm3̄m) appears above 700 °C. Several factors seem to be responsible for the increase in the temperature of the R3̄c → Pm3̄m phase transition in the reduced form of the La_0.95Ni _0.6Fe_0.4O_3-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 La _0.95Ni_0.6Fe_0.4O_3-δ at room temperature.