Structural and Luminescence Properties of Ca_1-xLa_xS (x=0-0.3)

The structure and photoluminescence of Ca_1-xLa_xS (x=0-0.3) were investigated. The samples were prepared by sulfurizing the mixture of CaCO₃and La₂O₃in the flux of Na₂CO₃-S or K₂CO₃-S, and by gas reaction with CS₂. By using different methods of preparation, defect structure and concentration could be controlled chemically. Ca_1-xLa_xS prepared in the Na₂CO₃-S flux has a wider solid solution range than that prepared in the K₂CO₃-S flux, and such a feature seems to be due to similar ionic size of Na⁺and Ca²⁺. As the substitution of La³⁺increases, the band gap of the host material decreases due to the increase of the lattice parameters, and the photoluminescence spectra of the Ca_1-xLa_xS shift to longer wavelengths. Since La³⁺ion itself is transparent to ultraviolet radiation, vacancies (V_Ca2+, V_S2+) and substituted ions (Na⁺_Ca2+, La³⁺_Ca2+) seem to be associated with luminescence centers. The acceptor levels of Na⁺_Ca2+and V_Ca2+are estimated to be about 1.1 and 0.5 eV above the valence band, respectively, and the donor levels of La³⁺_Ca2+and V_S2-to be about 1.8 eV below the conduction band. The emission bands observed at 500-580 nm suggest the recombination processes of donors with acceptors.