Tetrahedrally coordinated Co2+ in oxides and silicates: Effect of local environment on optical properties

The Co2+ ion in fourfold coordination provides d-d electronic transitions with the strongest optical density among oxides and silicates. For this reason, it is widely used in pigments and dyes to get blue shades detectable down to a very low cobalt concentration. Such a low-detection limit turns the Co2+ ion into a suitable probe to disclose the local ligand environment in a wide range of materials by means of optical spectroscopy. Even if extensively studied in organometallic complexes, an in-depth investigation of optical properties of Co2+ in tetrahedral coordination into oxidic structures is limited to some case-study in minerals and synthetic analogs (spinel, zincite, gahnite, willemite, calcium cobalt selenite). The present study represents an attempt to outline crystal structural (long-range metal oxygen distances, O-T-O bond angles, and distortion parameters by XRD) and optical parameters (10Dq, Racah B and C, band splitting by EAS) in 13 samples of oxides and silicates providing a wide set of different local fourfold coordination around Co2+ added as a dopant. Subtle variations of crystal field strength and interelectronic repulsion can be appreciated in gahnite, Ca-Sr-hardystonite, Ca-Sr-Ba-akermanite, willemite, Ba2MgSi2O7 melilite-related (where Co2+ substitutes Mg2+ or Zn2+ by 0.25-0.3 apfu) as well as in gehlenite and fresnoite (where Co2+ substitutes Al3+ and Ti4+, respectively, by 0.2 apfu due to charge mismatch). Results are compared with literature data about hibonite, spinel s.s., staurolite, yttrium garnets, and zincite. Spectral interpretation is not straightforward owing to the occurrence of different Co2+ bands: spin-allowed and spin-forbidden electronic transitions, two- or threefold split due to both lowering of point symmetry at the tetrahedron and spin-orbit coupling plus presumably vibronic transitions. Optical spectra vary significantly even for apparently small changes in the long-range Cola, arrangement as measured by XRD. The expected relationship between 10Dq and the mean Co-O distance is fulfilled, but the accommodation into small AlO4 sites in gehlenite (YAG and hibonite) implies a significant structural relaxation around the Co2+ ion. The threefold splitting of the spin-allowed T-4(1)(F) and T-4(1)(P) bands can be related to the angular distortion of the CoO, tetrahedra. Overall, changes of spectral features of tetrahedrally coordinated Co2+ can be attributed to different local arrangement of ligands with an effect correlated to the second nearest neighbors by the bond valence theory. This was disclosed contrasting 10Dq with the ratio of the observed and ideal bond valence sum for the polyhedra sharing oxygen with the Co-centered tetrahedron.