Influence of the genesis on the structural and hyperfine properties of Cr-substituted hematites

Several series of Cr-substituted hematites with Cr:Fe molar ratio up to 0.112 were prepared by forced hydrolysis of Fe3+ solutions and by thermal dehydroxylation of Cr substituted goethites. Powder X-ray diffraction was used in order to assess the structural characteristics of the whole series. Rietveld refinement of XRD data for samples obtained by forced hydrolysis indicated that the incorporation of Cr causes anisotropic changes in the a-lattice parameter and stacking faults. In hematite samples obtained from Cr-goethites, the cell-parameters decrease with the increase in Cr-content. In all cases, the particles were multidomainic, with sizes that varied between 0.13 ± 0.02 and 1.07 ± 0.05 μm. rin transition was detected in substituted samples with low Cr-content, but it was no longer present in samples containing μCr ≥ 4.3 ± 0.2 (μCr = 100 × [Cr] / [Cr] + [Fe] mol mol− 1). Mössbauer spectroscopy suggested that the AF phase of Cr-hematites obtained by forced hydrolysis of Fe(III) salts, did not present parallel spin alignment along the [111] direction. These solids presented an unusually low value for TM which was ascribed to the interplay of anisotropic lattice changes, small grain sizes, metal vacancy sites and structurally bound water. All samples presented spin orientation in the (111) basal plane in the WF phase. emperature variation was also explained by the concomitant contributions coming from Cr-for-Fe substitution and anisotropic lattice variations.