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 0.95 Ni 0.6 Fe 0.4 O 3−δ with temperature and oxygen partial pressure variation. Neutron diffraction demonstrates that at room temperature the A-site deficient nominal composition La 0.95 Ni 0.6 Fe 0.4 O 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 R 3̅ c (167), a = 5.4491(1) Å, β = 60.761(1)°, and V = 116.372(3) Å 3 . NiO secondary phase was refined to rhombohedral structure with space group R 3̅ m (No. 166), a = 2.9560(5) Å, β = 59.99(1)°, and V = 18.260(2) Å 3 . High-temperature phase transformation of the perovskite phase from rhombohedral ( R 3̅ c ) to cubic ( Pm 3̅ m ) symmetry was observed above 550 °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 La 0.95 Ni 0.6 Fe 0.4 O 3−δ, the cubic phase ( Pm 3̅ m ) appears above 700 °C. Several factors seem to be responsible for the increase in the temperature of the R 3̅ c → Pm 3̅ m phase transition in the reduced form of the La 0.95 Ni 0.6 Fe 0.4 O 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 0.95 Ni 0.6 Fe 0.4 O 3−δ at room temperature.