Optical and Structural Properties of RE³⁺-Doped KLnS ₂ Compounds

Rare-earth (RE= Sm, Pr, Tb, Tm) doped ternary sulfides having the general formulaKLn 0.995 RE0.005 S ₂(Ln= La, Gd, Lu) and undopedKLnS ₂were 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. TheKLnS ₂band edge at 300–330 nm was found and characteristicPr³⁺,Sm³⁺,Tb³⁺andTm³⁺~ 4 f -4 femission lines in the visible spectral range were observed. The charge transfer transitions in thePr³⁺- andSm³⁺-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.