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

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.