Structural relaxation around Cr3+ and the red-green color change in the spinel (sensu stricto)-magnesiochromite (MgAl2O4-MgCr2O4) and gahnite-zincochromite (ZnAl2O4-ZnCr2O4) solid-solution series

Optical absorption spectra of flux-grown single crystals in the spinet s.s.-magnesiochromite and gahnite-zincochromite solid solutions were recorded with the aim of exploring variations in local Cr-O bond distance as a function of composition. With increasing Cr contents, the crystals vary in color from pale red to intensely red to dark greenish. These variations are reflected in the optical spectra by the position and intensity of the two spin-allowed electronic d-d transitions in six-coordinated Cr3+ at similar to 18000 (v(1)) and 25 000 cm(-1) (v(2)). From the shift of the v(1) band position, a decrease in crystal field splitting, 10Dq, for six-coordinated Cr3+ with increasing Cr contents is evident in both solid-solution series. Based on published Cr-O bond distances for the CrO6 polyhedra in maanesiochromite and zincochromite of 1.995 and 1.991 angstrom, respectively, and applying the ligand field relationships, local Cr-O bond distances in gahnite and spinet with Cr contents at trace levels are determined to be 1.974(2) and 1.960(3) angstrom, respectively. These local Cr-O distances result in relaxation parameters (epsilon) equal to 0.69(2) and 0.60(3) for Cr-O bonds in the Mg(Al1-xCrx)(2)O-4 and Zn(Al1-xCrx)(2)O-4 series, respectively. However, the presently obtained Racah B values indicate increasing Cr-O bond covalency with increasing Cr3+ contents. This suggests that color changes and accompanying 10Dq variations may be due to variations in Cr-O bond covalency along the two solid-solution series, without or with very minor local Cr-O bond distance variation. Consequently, the epsilon values obtained from the present optical absorption spectra should be regarded as minimum values.