Structural and luminescence properties of Ca1-xLaxS (x=0-0.3)

Abstract

The structure and photoluminescence of Ca1-xLaxS (x=0-0.3) were investigated, The samples mere prepared by sulfurizing the mixture of CaCO3 and La2O3 in the flux of Na2CO3S or K2CO3S, and by gas reaction with CS2. By using different methods of preparation, defect structure and concentration could be controlled chemically, Ca1-xLaxS prepared in the Na2CO3S 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-x)Lax(S) shift to longer wavelengths, Since La3+ ion itself is transparent to ultraviolet radiation, vacancies (VCa2+, VS2+) and substituted ions (Na-Cn2+(+), La-Ca2+(3+)) seem to be associated with luminescence centers. The acceptor levels of Na-Ca2+(+) and VCa2+ are estimated to be about 1.1 and 0.5 eV above the valence band, respectively, and the donor levels of La-Ca2+(3+) and VS2- 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 accepters. (C) 1997 Academic Press.