Photoluminescence studies of layered transition metal phosphorus chalcogenides and their pyridine intercalation compounds

Ultraviolet photoexcitation of Cd₂P₂S₆ and Zn₂P₂S₆ single crystals produces a characteristic photoluminescence between 2.5 and 1.5 eV. Mn₂P₂S₆ exhibits only low energy emission due to transitions between localized d-orbital states. These results are consistent with the electronic band structure model for transition metal phosphorus chalcogenides with the general chemical formula M₂P₂X₆ (Cd, Zn, Mg, Mn, X = sulfur or selenium), which predicts highly localized states associated with the transition metal d orbitals and delocalized, band-like states derived from sulfur and phosphorus orbitals. When pyridine containing a small amount of water is intercalated into Cd₂P₂S₆, the excitation and the photoluminescence spectra are changed substantially. The photoluminescence intensity increases approximately tenfold and the position of the excitation edge is shifted to lower energy by 0.5 eV. Intercalation with anhydrous pyridine does not affect either the excitation spectrum or the photoluminescence spectrum of the host lattice.