Oxygen Stoichiometry in the Barium Ferrates; Its Effect on Magnetization and Resistivity

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Abstract
The barium ferrates comprise a series of compositions exhibiting a wide range of oxygen deficiency. Members of this series are isostructural with the high‐temperature polymorph of barium titanate which is hexagonal with space group C63/mmc. Polycrystalline specimens with varying oxygen contents, i.e., Fe4+ concentrations, were prepared by equilibration at oxygen pressures ranging from 0.2 to 2400 atm. The highest oxygen content achieved was BeFeO2.95 which had lattice parameters: a=5.674 Å, c=13.645 Å. This specimen exhibited unusual magnetic properties marked by an abrupt change in magnetization at 164°K. Above this temperature the material appeared to be ferrimagnetic. The peak value of the magnetization per gram, σg, is 13.8 emu at 164°K. At lower temperatures the compound is antiferromagnetic but exhibits a small residual moment. The transition appears to be first order with a thermal hysteresis of approximately 4°C between heating and cooling curves, and is accompanied by a discontinuity in the plot of resistivity vs reciprocal temperature. X‐ray diffraction patterns showed no distortion from hexagonal symmetry at temperatures below the transition. Specimens exhibiting greater oxygen deficiency had larger lattice parameters. In general, these showed a less abrupt magnetic transition which occurred at progressively lower temperatures.

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