D2d symmetry of CuInS2:Co2+ single crystals

CuInS2 and CuInS2:Co2+ single crystals were synthesized by chemical transport reaction using iodine as a transporting medium. The absorption spectra of CuInS2:Co2+ single crystals at 294 K showed two broad peaks due to 4A2(4F)→4T1(4F) and 4A2(4F)→4T2(4F) transitions of Co2+ ions in the Td symmetry. The 4A2(4F)→4T1(4F) transition peak had three sub-peaks induced by the first-order spin orbit coupling. The absorption spectra in the 4A2(4F)→4T1(4F) transition region at 4.7 K showed 12 peaks, which can be analyzed using the additional D2d symmetry and phonon replicas. This symmetry reduction was caused by the anion displacement, not by the tetragonal distortion of the unit cell. The optical energy gap of CuInS2:Co2+ single crystals decreased as the amount of Co2+ ions increased.