Optical absorption of Co²⁺ in MgIn₂S₄, CdIn₂S₄, and HgIn₂S₄ spinel crystals

The optical absorption spectra of cobalt-doped MgIn₂S₄, CdIn₂S₄, and HgIn₂S₄ single crystals grown by a chemical transport reaction were investigated at 7 K. The values of Dq and B calculated from the observed spectra show that the d electrons are more delocalized in the order of MgIn₂S₄, CdIn₂S₄, and HgIn₂S₄. Because this order corresponds to the decreasing order of the electronegativity differences between the metal and sulfur atoms of the host crystals, we conclude that as the metal-sulfur bonds in the host crystals are more covalent, the d electrons of the cobalt impurities are more spread out and the cobalt-sulfur bonds are also more covalent.