High-temperature phase transitions in the La0.25Sr0.75FeO3−δ solid solution with a perovskite structure

Phase transformations in a sample from the series La1−xSrxFeO3−δ (x = 0.75) with cubic symmetry and perovskite type structure were studied by high-temperature X-ray diffraction. In air, the solid solution was stable to the synthesis temperature (1200°C), despite the loss of some part of oxygen. Heating in vacuum led to a two-phase state at 900°C (or even at 600°C when a small amount of palmitic acid was added to the sample); the unit cell parameter changed abruptly. The amount of the new phase increased with temperature, and at 1000°C, the sample, as well as the starting phase, was monophase perovskite with cubic symmetry; i.e., an isosymmetric phase transition occurred, which was accompanied by an abrupt change in the cell volume. Cooling in vacuum to ∼800°C led to the ordering of the oxygen vacancies and the formation of a Grenier phase type structure. When the sample was heated again in vacuum, a reversible phase transition occurred from the Grenier phase type structure to a perovskite structure (again at ∼800°C).