Al hematites prepared by homogeneous precipitation of oxinates:: material characterization and determination of the Morin transition

A novel method for synthesis of aluminium hematites, based upon the homogeneous precipitation of Fe and Al oxinates in various proportions, is presented. The precursor precipitates are heated in air at 700 degreesC. X-ray diffraction, thermal analyses, BET, FTIR, optical reflection analysis, TEM and Mossbauer spectroscopy at room temperature and 80 K of the resulting products indicate that single-phase hematites are formed with structural Al substitution of up to 10 at%. Interestingly. the particle size ( > 100 nm) is not substantially reduced by the Al content. Although it remains difficult to obtain a homogeneously distributed Al substitution in the final hematite, this processing fine offers a unique opportunity to separate the effects of grain size and Al substitution on the Morin transition temperature (T-M) of Al hematite. From the comparison between the present hematites and a series of Al-substituted hematites with lepidocrocite as precursor, it could be shown that the effect on T-M, associated with a change of a factor 10 in grain size. is about 1/3 of the effect caused by a change of 10 in the degree of substitution. Finally, it is suggested that proper thermal treatments under different conditions of the same precursors are likely to produce spinel phases.