The magnetic and structural properties of a series of lanthanum based transition metal perovskites

Abstract

Fe-doped lanthanum–nickel perovskite was prepared using several methods based on co-precipitation and sol–gel technologies. A simple complexing-precipitation reaction, involving the addition of tetramethyl ammonium hydroxide to aqueous nitrate solutions, gave an oxy-hydroxide gel. On ageing this produced a single-phase perovskite product at low temperatures (about 1073 K) for LaNiO 3. However, the formation of the Fe-doped materials yielded a far more complex oxide system with three different perovskite phases being identified. This material was shown to exhibit super-paramagnetism. Mössbauer spectroscopy performed on Fe- and Eu-doped samples showed that all metal cations are trivalent. In agreement with previous work [A. Reller, T. Williams, Chem. in Britain (1989) 1227] it was found that the structure was a rhombohedrally distorted perovskite. Lattice parameters were determined accurately by Rietveld analysis. Mössbauer spectroscopy was used to monitor changes in the magnetic properties of these materials on addition of iron (between 5 and 100 metal mol%). Up to iron loadings of 30% the system exhibits only super-paramagnetic properties. Above this level there is a complex mixture of super-ferromagnetic and ferromagnetic components. Above 90% only ferromagnetic properties are displayed. These observations are not due to the precipitation of spinel-type phases but, rather, have been rationalised in terms of the precipitation of new distorted perovskite phases and electronic modifications dominated by local interactions and long range co-operative effects between these local centres.