The luminescence of europium(III) and cerium(III) in calcium sulfate: Activators with an effective charge

The luminescence and the quantum efficiency of the phosphors CaSO{sub 4}:Eu{sup 3+} and CaSO{sub 4}:Ce{sup 3+} are reported. The Eu{sup 3+} phosphor shows a very weak luminescence under charge-transfer excitation, the quantum efficiency amounting to about 10%. Codoping of the Eu{sup 3+} phosphor with P{sup 5+} or Na{sup +} as charge compensators enhances the luminescence efficiency. This is ascribed to a decrease of the concentration of vacancies in the lattice. The phosphor CaSO{sub 4}:Ce{sup 3+} shows a very efficient ultraviolet luminescence. The Stokes shift of the d-f emission amounts to about 800 cm{sup {minus}1}. Vibrational structure is observed in the emission and excitation spectra. It is shown that the effective charge on the activator ion has a different influence on the relaxation of the opposite-parity excited states of Eu{sup 3+} and Ce{sup 3+}.