Influence of co-doping with Cl- on the luminescence of CaS:Eu2+

In this work, CaS:Eu2+ and CaS:Eu2+,Cl- samples are prepared by solid chemical reaction. Excitation and emission spectra are investigated. The emission band is shifted to the red from 655 nm for CaS:Eu2+ to 670 nm for CaS:Eu2+,Cl-. The absorption bands of CaS:Eu2+ are resolved in the excitation spectrum, peaking at 280, 365, 460, and 570 nm. In contrast, the excitation (absorption) spectrum of CaS:Eu2+, Cl- becomes a very broad platform in the range of 250 to 640 nm. The changes in the optical spectra are ascribed to the additional field splitting of the 4f(6)5d state of Eu2+ in the distorted lattice field by incorporation of Cl-. which replaces one of the six ligand ions of S2- and lowers the symmetry of the Eu site from O-h to C-4v. The red shift is a resultant effect of field splitting and the Stokes shift of Eu2+ in the two lattice environments. In addition, an afterglow is observed in the sample of CaS:Eu2+,Cl-. The traps, which are responsible for the afterglow at room temperature, are related to the charge defects created by incorporation of the anion Cl-. (C) 2000 The Electrochemical Society. S0013-4651(00)01-058-2. All rights reserved.