Formation and UV absorption of cerium, europium and terbium ions in different valencies in glasses

Cerium, europium and terbium ions can exist in different valencies in glasses. The formation and ultraviolet (UV) absorption features of the ions were studied in a fluoride phosphate (FP) and two phosphate glasses. Various melting conditions and X-ray irradiation were applied to change the redox states of the ions. Band separation of the UV absorption spectra was carried out to reveal the components and to determine their spectroscopic properties. The UV absorption spectra of the lower valent Eu2+, Tb3+ and Ce3+ ions are due to 4f-5d transitions, which are split in several bands by the local field around the rare-earth (RE) ions. The crystal field splitting of the trivalent ions differs from the one of the divalent ion. The UV absorption spectra of the higher valent Eu3+, Tb4+ and Ce4+ ions are caused by charge transfer (CT) transitions from oxygen and fluorine to the RE ions. The positions and oscillator strengths of the 4f-5d and CT transitions are studied in dependence on the RE and glass type. Furthermore, the ligand field strength of Eu2+ ions is investigated. Redox tendency, site symmetry and charge of the RE ions are important factors considering the influence of the RE type. Different polarizability and electron donor power of the ligands as well as different RE site symmetry in the glasses cause the compositional dependence of the transition properties. (C) 2000 Elsevier Science B.V. All rights reserved.