β-In 2 S 3 and β-In 2 S 3 :Co 2+ single crystals were grown by the chemical transport reaction method using In 2 S 3 , S, and ZnS as starting materials and (ZnCl 2 + I 2 ) as a transport agent. The single crystals crystallized into a tetragonal structure. The indirect optical energy band gaps of the single crystals at 298 K were found to be 2.240 eV and 1.814 eV for β-In 2 S 3 and β-In 2 S 3 :Co 2+ , respectively. The direct optical energy band gaps were found to be 2.639 eV and 2.175 eV for β-In 2 S 3 and β-In 2 S 3 :Co 2+ , respectively. Impurity optical absorption peaks were observed for the β-In 2 S 3 :Co 2+ single crystal. These impurity absorption peaks were assigned, based on the crystal field theory, to the electron transitions between the energy levels of the Co 2+ ion sited in T d symmetry.

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CuAl 2 S 4 single crystals were grown with the constituent elements copper, aluminium and sulphur (purity 6N) as the starting material by the chemical transport reaction method using iodine as a transport agent. Layer-type colourless transparent single crystals were obtained, and the crystal structure of the single crystals was found to be cubic with the lattice constant a = 9.954 Å. The energy bandgap of the CuAl 2 S 4 single crystal was revealed to be direct bandgap, and the optical energy gap was found to be 4.10 eV at 10 K.

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.

MnAl 2 S 4 and MnAl2Se4 single crystals were grown by the chemical transport reaction method. Optical energy gaps of the MnAl2S4 and MnAl2Se4 single crystals were 3.75 and 3.21 eV, respectively, at 300 K. Emission peaks due to donor-acceptor pair recombinations were observed at 450 and 603 nm in the MnAl2S4 single crystal and at 488 and 655 nm in the MnAl2Se4 single crystal. Optical absorption peaks and emission peaks described as appearing due to Mn2+ ion sited in Td symmetry were observed at 414, 450, 482, and 527 nm in the MnAl2S4 single crystal and at 416, 455, 488, and 532 nm in the MnAl2Se4 single crystal.