Optical Properties and Local Structure of Eu3+ Ions in Sol−Gel TiO2−SiO2 Glasses

TiO 2 −SiO 2 glasses containing Eu 3+ ions have been prepared by the sol−gel method. The energy transfer from the O−Ti charge-transfer band to the Eu 3+ ions was observed. The intensity ratio of 5 D 0 → 7 F 2 to 5 D 0 → 7 F 1 increases and the spectrum band of the 5 D 0 → 7 F 0 transition shifts to the lower energy side as the TiO 2 concentration increases, indicating the formation of Eu 3+ −O 2- −Ti 4+ bonding. At the same time, the emission intensity and the average decay time increase, revealing that the incorporation of TiO 2 can reduce the extent of the Eu 3+ clusters. Fluorescence line-narrowing spectra indicate that the Eu 3+ ions have two different environments, one of which exhibits a line-narrowing effect. The second-order crystal-field parameters show that the coordinating oxygen ions of site II are closer to the Eu 3+ ions at the higher excitation energy. The analysis on the local structure of the Eu 3+ ions suggests that the Ti 4+ ions at site I occupy tetrahedral sites within the silica network, while some Ti 4+ ions at site II occupy octahedral sites. The temperature dependence of the excitation spectra confirms that the 7 F 1 level can be thermally populated in part and lower energy photons via thermally excited Eu 3+ ions can make some electrons populate the 5 D 0 level.