Valence State of Europium in CaS Prepared Using Alkaline Polysulfide Flux

Red calcium sulfide phosphors were prepared using the flux method. Light yellow or pink body colour was obtained for powders synthesized from Na2S x or K2S x , respectively. The pink colour is related to the absorption band of Eu2+ in the visible region, whereas this band is absent in the yellow powder. Diffused absorption spectra and time-resolved photoluminescent spectra show that europium exists mainly in the trivalent or divalent form for yellow or pink powder, respectively. The presence of Eu3+ in the CaS matrix implies the co-insertion of the flux alkaline ions to respect the electrical neutrality. Because of their similar sizes, Na+ is easily substituted for Ca2+, while K+ can be inserted only in small amounts. Only Eu2+ emission is observed, and the efficiency is nearly the same in both cases. However, the quenching concentration is about one order of magnitude higher (10-2 mol Eu) for the pink powder. The high efficiency observed for the yellow powder may be due to the capture of one electron by Eu3+ leading to excited Eu2+ and to light emission when electron and hole are recombined.