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 Mo ̈ ssbauer spectroscopy. With the
exception of iron-poor samples (3–12 mol-% MgFe2O4),
optical absorption and Mo ̈ ssbauer 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 cm1 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 Mo ̈ ssbauer spectra. Profound
composition-related changes are observed in the
Mo ̈ ssbauer 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 a0 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.
