Synthetic and natural chromium-bearing spinels: an optical spectroscopy study

Four samples of synthetic chromium-bearing spinels of (Mg, Fe2+)(Cr, Fe3+)(2)O-4 composition and four samples of natural spinels of predominantly (Mg, Fe2+)(Al, Cr)(2)O-4 composition were studied at ambient conditions by means of optical absorption spectroscopy. Synthetic end-member MgCr2O4 spinel was also studied at pressures up to ca. 10 GPa. In both synthetic and natural samples, chromium is present predominantly as octahedral Cr3+ seen in the spectra as two broad intense absorption bands in the visible range caused by the electronic spin-allowed (4) A (2g) -> (4) T (2g) and (4) A (2g) -> (4) T (1g) transitions (U- and Y-band, respectively). A distinct doublet structure of the Y-band in both synthetic and natural spinels is related to trigonal distortion of the octahedral site in the spinel structure. A small, if any, splitting of the U-band can only be resolved at curve-fitting analysis. In all synthetic high-chromium spinels, a couple of relatively narrow and weak bands of the spin-allowed transitions (4) A (2g) -> (2) E (g) and (4) A (2g) -> (2) T (1g) of Cr3+, intensified by exchange-coupled interaction between Cr3+ and Fe3+ at neighboring octahedral sites of the structure, appear at similar to 14,400 and similar to 15,100 cm(-1). A vague broad band in the range from ca. 15,000 to 12,000 cm(-1) in synthetic spinels is tentatively attributed to Cr-IV(2+) + Cr-VI(3+) -> Cr-IV(3+) + Cr-VI(2+) intervalence charge-transfer transition. Iron, mainly as octahedral Fe3+, causes intense high-energy absorption edge in near UV-range (ligand-metal charge-transfer O2- -> Fe3+, Fe2+ transitions). As tetrahedral Fe2+, it appears as a strong infrared absorption band at around 4,850 cm(-1) caused by electronic spin-allowed (5) E -> (5) T (2) transitions of Fe-IV(2+). From the composition shift of the U-band in natural and synthetic MgCr2O4 spinels, the coefficient of local structural relaxation around Cr3+ in spinel MgAl2O4-MgCr2O4 system was evaluated as similar to 0.56(4), one of the lowest among (Al, Cr)O-6 polyhedra known so far. The octahedral modulus of Cr3+ in MgCr2O4, derived from pressure-induced shift of the U-band of Cr3+, is similar to 313 (50) GPa, which is nearly the same as in natural low-chromium Mg, Al-spinel reported by Langer et al. (1997). Calculated from the results of the curve-fitting analysis, the Racah parameter B of Cr3+ in natural and synthetic MgCr2O4 spinels indicates that Cr-O-bonding in octahedral sites of MgCr2O4 has more covalent character than in the diluted natural samples. Within the uncertainty of determination in synthetic MgAl2O4 spinel, B does not much depend on pressure.