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-åkermanite, 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 CoO₄ 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 ⁴T₁(F) and ⁴T₁(P) bands can be related to the angular distortion of the CoO₄ 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.

The structural relaxation around the Co²⁺ ion along the gahnite (ZnAl₂O₄)-Co-aluminate (CoAl₂O₄) join was investigated by a combined X-ray diffraction (XRD) and electronic absorption spectroscopy (EAS) approach. Monophasic spinel samples (Zn_1−yCo_yAl₂O₄ with y = 0, 0.25, 0.5, 0.75, and 1 apfu) were obtained through solid-state reaction (1300 °C with slow cooling). The cobalt incorporation induces a linear increase of the unit-cell parameter (a) accompanied by an increasing inversion parameter (up to 0.07) so that the Co²⁺ for Al³⁺ substitution in the octahedral site is, at a first approximation, the cause of the lattice expansion. However, a careful consideration of T-O distances highlights the role played by an enhanced covalence degree of Zn-O bonds. The optical spectra are characterized by the occurrence of electronic transitions of Co²⁺ in tetrahedral coordination affected by a strong spin-orbit coupling, causing a threefold splitting of spin-allowed bands. Further complications stem from mixing of quadruplet and doublet states (leading to a consistent intensity gain of spin-forbidden bands) and vibronic effects (producing intense sidebands). Crystal field strength goes from 4187 to 4131 cm⁻¹ with increasing cobalt amount, while the Racah B parameter is in the 744–751 cm⁻¹ range (C ~3375 cm⁻¹). To achieve a reliable estimation of the local Co-O distance, the tetrahedral distance evolution was recast to eliminate the effects of the inversion degree. By this way, a relaxation coefficient as low as ɛ = 0.47 was obtained, i.e., significantly smaller than literature data for other spinel systems. The gahnite-Co-aluminate join seems to be constrained by the strong preference of Zn²⁺ for the tetrahedral site in which its enhanced covalency can be exerted, limiting the cation exchange between tetrahedral and octahedral sites as well as the lattice flexibility.