Structural investigations of sol-gel derived silicate gels using Eu3+ ion-probe luminescence

Undoped and Eu3+-doped CaF2-SiO2 gels were prepared by the sol-gel method and their optical properties have been studied. The UV-VIS-NIR absorption and photoluminescence spectra have shown the bands typical for the Eu3+ ions transitions. When the Eu-doped gel is annealed at temperatures up to 800 °C (i.e. above the CaF2 crystallisation peak at ∼460 °C) the photoluminescence spectra intensity increase, the 590 nm (5D0→7F1) and 620 nm (5D0→7F2) luminescence bands become comparable and a structuring of the 620 nm band is observed. The phonon sidebands peaks associated with the 5F0→7D2 transition of the Eu3+ ion were observed at around 1000 and 620 cm−1 and have been assigned to the Si-O and Ca-O bonds, respectively. A phonon sideband signal in the range of 300-400 cm−1 was attributed to Ca-F bonds in the precipitated CaF2 phase. From the optical absorption, photoluminescence and phonon sidebands spectra we have concluded that in the gels annealed at 800 °C, the Eu3+ ions are incorporated into the silica network and in the precipitated CaF2 phase.