A-site deficiency, phase purity and crystal structure in lanthanum strontium ferrite powders

Lanthanum strontium ferrite (LSF) powders of average composition of (La1−x′ Srx′) y′FeO3+δ, were fabricated over a range of average strontium composition between 0.2 ≤ x′ ≤ 0.5 and average A-site occupancy between 0.8 ≤ y′ ≤ 1.0. Samples that were originally determined to be perovskite phase pure using X-ray diffraction with CuKα radiation were found to have significant amounts of non-perovskite phases when evaluated using high-energy synchrotron radiation. As-fabricated samples with nominal A-site deficiency, y′ < 1, contained a hexaferrite phase. When treated at 955 °C in pO2 = 10− 10 atm, those samples contained magnetite. The actual composition of the perovskite phase was corrected to account for the presence of the second phases through mass balance calculations. As a result, the actual A-site deficiency of the perovskite phase was much lower than the average value of the bulk powder. For as-fabricated powders with x′ < 0.4, it was determined that the A-site deficient LSF perovskite phases were metastable. At equilibrium, a mixture of A-site stoichiometric perovskite and hexaferrite phases was present. The refined perovskite crystal structures and unit cell volumes were consistent with literature trends.