Valencies of Eu in CaS:Eu, La

Photoluminescence of La and Eu codoped CaS prepared in sodium and potassium polysulfide flux was studied. The emission spectra of CaS:Eu prepared in both fluxes show a broad band at 650 nm due to the f-d transition of Eu²⁺. When potassium polysulfide flux is used, the emission from Eu²⁺decreases and the line emission of Eu²⁺appears with concentration of codoped La³⁺. However, when sodium polysulfide flux is used, clear line emission spectra of Eu³⁺were observed, in addition to the broad band spectrum from recombination process of defects and vacancies in the host matrix. For the first time at room temperature, sharp line spectrum of Eu³⁺is observed and this is a strong indication of La³⁺-Eu³⁺pairing in CaS:Eu, La. EPR intensity of Eu²⁺increases with Eu concentration and decreases with La³⁺concentration in CaS:Eu, La prepared in potassium polysulfide flux. However, the result of EPR study suggests that only Eu³⁺is doped when CaS:Eu, La is prepared in sodium polysulfide flux.