Synthesis, physical and optical properties, and electronic structure of the rare-earth oxysulfides Ln2O2S (Ln=Sm, Eu)

The compounds Ln2O2S (Ln=Sm, Eu) were prepared from the respective oxides by partial sulfurization under flowing CS2/Ar/O2 atmosphere. The main feature of the structure is the packing of S2− and O2− anions with Ln3+ located in the center of a capped octahedron. Electrical conductivity measurements on pressed pellets showed that both phases are highly resistive with conductivities lower than 10−8 S cm−1 at room temperature. The optical bandgaps were assessed at 4.2 and 4.4 eV for Sm2O2S and Eu2O2S, respectively. Electronic structure calculations were performed by means of extended Hückel tight-binding method. The insulating properties are in good agreement with the results predicted by theoretical calculations.