Abstract
The influence on the spinel structure of Fe3+ → Cr substitution was studied in flux-grown synthetic single crystals of the magnesiochromite–magnesioferrite (MgCr2O4–MgFe2O4) solid solution series. Samples were analysed by single-crystal X-ray diffraction, electron microprobe analyses, optical absorption and Mössbauer spectroscopy. With the exception of iron-poor samples (3–12 mol-% MgFe2O4), optical absorption and Mössbauer spectra show that iron occurs almost exclusively as trivalent Fe in the present samples. A very intense and broad absorption band at ca 7,800 cm−1 dominates the optical absorption spectra of samples with higher Fe-contents. The appearance of this band is related to a distinct structural disorder of Fe3+ and a development of magnetic ordering as demonstrated by Mössbauer spectra. Profound composition-related changes are observed in the Mössbauer spectra, which are magnetically unsplit in the range 2–41 mol-% magnesioferrite, but become magnetically split in the range 59–100 mol-% magnesioferrite. Structural parameters a 0 and M–O increase with magnesioferrite content and inversion degree, while u and T–O decrease. Our study confirms the previously reported (Lavina et al. 2002) influence of Fe3+ at the M site on T–O bond lengths in the spinel structure.
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Acknowledgments
DL thanks the Swedish Museum of Natural History and the financial support through HIGH LAT resources (project number HPRI-CT-2001.00125) within the European Community—Access to Research Infrastructure Action of the Improving Human Potential Programme. This work was supported with MURST and Trieste University grants to FP (Cristallochimica e reazioni di scambio cationico in spinelli, olivine e pirosseni; COFIN 2001). The Italian C.N.R. financed the installation and maintenance of the microprobe laboratory at the University of Padova. R. Carampin, L. Tauro and L. Furlan are kindly acknowledged for technical support. HS and UH acknowledge financial support from the Swedish Research Council (VR). G.B. Andreozzi and an anonymous reviewer are kindly acknowledged for their suggestions.
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Lenaz, D., Skogby, H., Princivalle, F. et al. The MgCr2O4–MgFe2O4 solid solution series: effects of octahedrally coordinated Fe3+ on T–O bond lengths. Phys Chem Minerals 33, 465–474 (2006). https://doi.org/10.1007/s00269-006-0093-y
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DOI: https://doi.org/10.1007/s00269-006-0093-y