Anomalous temperature dependence of the optical energy gaps and the optical absorption for undoped and Co2+-doped Cd4GeS6 single crystals

Cd4GeS6 and Cd4GeS6:Co2+ single crystals were grown by using the chemical transport reaction method. The single crystals were crystallized into a monoclinic structure. The optical energy gaps of the Cd4GeS6 and the Cd4GeS6: Co2+ single crystals were found to be 2.566 eV and 2.450 eV, respectively, at 293 K. In the temperature dependence of the optical energy gap, an anomalous property that the optical energy gap decreased with decreasing temperature from 73 K to 10 K was observed. This anomalous temperature dependence was shown to be due to the anomalous temperature dependence of the lattice constant a of the single crystals. Impurity optical absorption peaks were observed in the Cd4Ge S6:Co2+ single crystal and were shown to be due to the electron transitions between the energy levels of the Co2+ ion sited at the pseudo-Td symmetry point.