Optical properties and local structure of Eu3+ ions in sol-gel TiO2_SiO2 glasses

TiO2-SiO2 glasses containing Eu3+ ions have been prepared by the sol-gel method. The energy transfer from the O-Ti charge-transfer band to the Eu3+ ions was observed. The intensity ratio of D-5(0)-->F-7(2) to D-5(0)-->F-7(1), increases and the spectrum band of the D-5(0)-->F-7(0) transition shifts to the lower energy side as the TiO2 concentration increases, indicating the formation of Eu3+-O-2(-)-Ti4+ bonding. At the same time, the emission intensity and the average decay time increase, revealing that the incorporation of TiO2 can reduce the extent of the Eu3+ clusters. Fluorescence line-narrowing spectra indicate that the Eu3+ 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 11 are closer to the Eu3+ ions at the higher excitation energy. The analysis on the local structure of the Eu3+ ions suggests that the Till ions at site I occupy tetrahedral sites within the silica network, while some Till ions at site 11 occupy octahedral sites. The temperature dependence of the excitation spectra confirms that the 7171 level can be thermally populated in part and lower energy photons via thermally excited Eu3+ ions can make some electrons populate the D-5(0) level.