A-site deficiency, phase purity and crystal structure in lanthanum strontium ferrite powders
T. Strikera, , , J.A. Ruuda, Y. Gaoa, W.J. Hewarda and C. Steinbruchelb
aGE Global Research Center, One Research Circle, Niskayuna, NY 12309, United States bDepartment of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180, United States Received 25 March 2007; revised 18 June 2007; accepted 18 June 2007. Available online 12 July 2007.
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.