Valencies of Eu in CaS:E, 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 Bus 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 Eu3+. (C) 1998 Academic Press.