Crystallisation of hexagonal M ferrites from a stoichiometric sol-gel precursor, without formation of the alpha.-BaFe2O4 intermediate phase

Single phase BaM (BaFe12O19) and SrM (SrFe12O19) ferrites were made from stoichiometric aqueous sol-gel precursors, and two different types of precursor sot were used, containing either halide or nitrate anions. It was found that halides were retained in the ferrite precursors to over 900 degreesC and delayed the formation of the M ferrite phase. This resulted in the formation of M ferrite at a temperature up to 200 degreesC higher for the halide-derived samples, compared to the nitrate-derived samples. Single phase halide-derived BaM and SrM were formed at 1000 and 850 degreesC, respectively, while the nitrate-derived precursors formed single phase BaM and SrM at 750 and 700 degreesC, respectively. No alpha-BaFe2O4, gamma-Fe2O3 or any other ferrite intermediate was observed in the BaM precursor powder, the M ferrite forming directly from alpha-Fe2O3 and barium salts. This indicated that the formation of alpha-BaFe2O4 or gamma-Fe2O3 is not necessary for the formation of BaM ferrite. (C) 2002 Elsevier Science BA. All rights reserved