Optical and Structural Properties of ${\rm RE}^{3+}$ -Doped ${\rm KLnS} _{2}$ Compounds

Rare-earth ( RE=Sm, Pr, Tb, Tm) doped ternary sulfides having the general formula KLn_0.995RE_0.005S₂ ( Ln=La, Gd, Lu) and undoped KLnS₂ were synthesized in the form of crystalline hexagonal platelets by a chemical reaction in an electric resistance furnace under the flow of hydrogen sulfide. X-ray powder diffraction confirmed only a single phase of the rhombohedral lattice system (space group R-3 m). Time-resolved luminescence spectroscopy was employed to investigate radioluminescence, photoluminescence, decay kinetics and absorption spectra. The KLnS 2 band edge at 300-330 nm was found and characteristic Pr³⁺, Sm³⁺, Tb³⁺ and Tm³⁺ 4 f -4 f emission lines in the visible spectral range were observed. The charge transfer transitions in the Pr³⁺- and Sm³⁺-related excitation spectra in the range 330-370 nm are described and the eficient energy transfer from the host to the emission centers is revealed. We discuss the potential for application of these materials in white LED solid-state lighting and X-ray phosphors.