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

Fe-doped lanthanum-nickel perovskite was prepared using several methods bas ed on co-precipitation and sol-gel technologies. A simple complexing-precip itation reaction, involving the addition of tetramethyl ammonium hydroxide to aqueous nitrate solutions, gave an oxyhydroxide gel. On ageing this prod uced a single-phase perovskite product at low temperatures (about 1073 K) f or LaNiO3. However, the formation of the Fe-doped materials yielded a far m ore complex oxide system with three different perovskite phases being ident ified. This material was shown to exhibit super-paramagnetism. Mossbauer sp ectroscopy performed on Fe- and Eu-doped samples showed that all metal cati ons are trivalent. In agreement with previous work [A. Reller, T. Williams, Chem. in Britain (1989) 1227] it was found that the structure was a rhombo hedrally distorted perovskite. Lattice parameters were determined accuratel y by Rietveld analysis. Mossbauer spectroscopy was used to monitor changes in the magnetic properti es of these materials on addition of iron (between 5 and 100 metal mol%). U p to iron loadings of 30% the system exhibits only super-paramagnetic prope rties. Above this level there is a complex mixture of super-ferromagnetic a nd ferromagnetic components. Above 90% only ferromagnetic properties are di splayed. These observations are not due to the precipitation of spinel-type phases but, rather, have been rationalised in terms of the precipitation o f new distorted perovskite phases and electronic modifications dominated by local interactions and long range co-operative effects between these local centres. (C) 1999 Elsevier Science S.A. All rights reserved.