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Melt-grown crystals of HgS and HgS : Co2+ were used to measure their optical absorption spectra. The optical energy band gaps of these crystals were 2.030 eV and 1.870 eV at room temperature, respectively. The impurity optical absorption peaks of Co2+ were observed at 4030, 5988, 12285, 12672, and 12905 cm−1. These peaks can be attributed to the electronic transitions between the split energy levels of Co2+ ion located at the Td symmetry site, where the crystal field, Racah, and spin-orbit coupling parameters were given by Dq = 403, B = 427, and Λ = −155 cm−1, respectively.

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Cd1-x Co x In2Se4 (x=0.0, 0.01, 0.05, 0.10, 0.50) single crystals were grown by the vertical Bridgman technique. The grown single crystals have a pseudocubic structure. The optical energy gap of these single crystals has an indirect energy gap whereas its value is decreased with increases in composition x. The optical absorption peaks due to cobalt atoms are observed at each composition. It is suggested that the substituted cobalt atoms for cadmium atoms are located at T d symmetry of the CdIn2Se4 host lattice as Co2+ ions by the static crystal field theory. These peaks can be attributed to the electronic transitions between the split electron energy levels of Co2+ ions.