The influence on the structure of Fe2+, Mg
substitution was studied in synthetic single crystals
belonging to the MgCr2O4–FeCr2O4 series produced by
flux growth at 900–1200 C in controlled atmosphere.
Samples were analyzed by single-crystal X-ray diffraction,
electron microprobe analyses, optical absorption-,
infrared- and Moessbauer spectroscopy. The Moessbauer
data show that iron occurs almost exclusively as IVFe2+.
Only minor Fe3+ (<0.005 apfu) was observed in samples
with very low total Fe. Optical absorption spectra
show that chromium with few exceptions is present as a
trivalent cation at the octahedral site. Additional
absorption bands attributable to Cr2+ and Cr3+ at the
tetrahedral site are evident in spectra of end-member
magnesiochromite and solid-solution crystals with low
ferrous contents. Structural parameters a0, u and T–O
increase with chromite content, while the M–O bond
distance remains nearly constant, with an average
value equal to 1.995(1) A corresponding to the Cr3+
octahedral bond distance. The ideal trend between
cell parameter, T–O bond length and Fe2+ content
(apfu) is described by the following linear relations:
a0=8.3325(5) + 0.0443(8)Fe2+ (A) and T–
O=1.9645(6) + 0.033(1)Fe2+ (A) Consequently, Fe2+
and Mg tetrahedral bond lengths are equal to 1.998(1) A
and 1.965(1) A, respectively.
