A detailed comparative study of the optical properties of two rare-earth (RE) sesquisulfide compounds γ−Ce2S3 and γ−Gd2S3, shows that if the two compounds have about the same main absorption edge corresponding to band-to-band S 3p-RE 5d transitions, Ce2S3 exhibits an additional absorption band at lower energies. This band, which is ascribed to the presence of Ce 4f states at the top of the valence band, explains the difference in color between the two compounds.

Optical and structural properties of chemically deposited CdS films have been characterized by using spectroscopic ellipsometry and electron transmission microscopy. Three representative cases have been studied using grown metastable cubic film and two films annealed at 400 °C (30 min) with and without a CdCl2 pretreatment (2 min dip in a CdCl2-saturated methanol solution). Without the pretreatment, the film remains at an intermediate state between the cubic and the stable hexagonal structure. With the pretreatment, a full transition to the hexagonal structure along with a strong recrystallization takes place. Full cubic or hexagonal films present high band gap values (2.44−2.46 eV), whereas band gap narrowing by ≈0.1 eV is observed in the second case. This effect is shown to be associated to a spatially modulated structure with a high density of stacking faults. This structure can be identified as a polytype cubic/hexagonal structure, which is a metastable intermediate in the phase transition process. Possible origins of the associated band gap lowering as order/disorder transition or stress effects are discussed.