Eu²⁺ luminescence in alkali sulphate lattices

Abstract

Results of optical absorption and fluorescence studies on europium-doped sulphates of caesium, rubidium and potassium are reported. The optical absorption spectra showed a band in the 5.2 to 5.3 eV region in each of the three phosphors. Additional weak shoulders in the lower energy region were also observed. These absorption bands are attributed to the 4f⁷ → 4f⁶ 5d transitions of europium which has entered the lattices predominantly in its divalent state. Fluorescence measurements also showed the host-dependent emission of Eu²⁺ in the 2.73 to 3.14 eV region with corresponding excitation spectra peaking in the 3.68 to 3.86 eV region. All these emission and excitation bands are assigned to Eu²⁺ ions due to transitions between the levels of 4f⁷ configuration and the excited mixed configuration of 4f⁶5d. The presence of trivalent europium could not be detected by luminescence studies in any of the three sulphate lattices.