Red and orange calcium sulfide phosphors (CaS:Eu and CaS:Mn) were prepared by the flux method. Eu2+ and Mn2+ show strong concentration quenching at 0.1 and 0.8% respectively. Use of the potassium polysulfide flux leads to efficient phosphors; their efficiency reaches 60 and 100% of Y2O2S:Eu, respectively. Calcium sulfide codoped with Eu and Mn shows a strong increase of Eu2+ in addition to Mn2+ emission. The colour obtained varies between red and orange depending on the concentration rates of Mn and Eu. A luminous efficiency of about 86% was reached for CaS:Eu(0.08%), Mn(0.3%). The decay characteristics of Mn2+ in CaS:Mn, Eu show that the fast part depends on the Eu2+ content. On the other hand, the Eu2+ decay became slower in the presence of Mn2+. These observations support the resonant mechanism of the energy transfer from Mn2+ to Eu2+.

CaS:Eu phosphors were synthesized using the flux method. ESR and Mössbauer spectroscopies show the coexistence of Eu3+ and Eu2+. Eu3+ and Eu2+ are mainly observed in phosphors obtained with Na2S z ((Na) phosphors) and K2S z fluxes ((K) phosphors), respectively. The Eu2+ fraction in (K) phosphors varies with total Eu content and temperature of synthesis. Under reducing thermal treatment, part of the Eu3+ is converted into Eu2+ in (Na) phosphors, and also in (K) phosphors with Eu content ≤10-3 mole. For the latter phosphors, addition of chloride ions in the flux enhances the Eu2+ level, and after reducing thermal treatment practically all europium ions are in the Eu2+ state. In similar conditions, (K) phosphors with more than 10-2 mole Eu exhibit nearly no Eu2+ level variation, and (Na) phosphors undergo a separation of phases to obtain CaS:Eu3+ and CaS:Eu3+Eu2+. Interpretation of the various features on the basis of Na (K), Eu codoping and dopant-induced lattice distortions is proposed. It is suggested that (K) phosphors are partially oxygenated, with Eu3+ ion charge balanced by K+ ions and/or Ca2+ vacancies.

Multiple-quantum transitions in the electron paramagnetic resonance spectrum of naturally doped ( N 10 ppm) CaS : Mn" powder are reported. Considerable resolution enhancement is achieved because of the lack of strain broadening of the EPR lines, due to the very good crystallinity of the powder. Multiple-quantum transitions are identified by their linewidth, their saturation behavior, and their magnetic field position. Two-, three-, and five-quantum transitions were identified, but not the four-quantum transitions. A line at g = 1.983 is assigned to Cr"' impurities at low concentration, which also exhibits a two-quantum transition at high microwave power.Cet article traite de ]'etude de transitions RPE multi-quanta dans des poudres de CaS:Mn" naturellement dopees. Les spectres presentent une grande resolution due a l'absence d'elargissement des raies RPE sous l'effet de contraintes dues au desordre, grLce a la tris bonne cristallinite des poudres. Les transitions multi-quanta sont identifiees par leur largeur, leur comportement en saturation et leur position en champ magnetique. Des transitions a deux, trois et cinq quanta ont ete identifiees, mais pas de transitions a quatre quanta. Une raie a g = 1.983 est attribuee a des impuretes Cr"' en faible concentration, qui montrent aussi une transition a deux quanta a forte puissance microonde.