FTIR spectroscopy to investigate the role of fluorine on the optical properties of pure and rare earth-doped sol–gel silica

FTIR spectroscopy is applied to monitor the effects caused by fluorinated precursors (1–10 mol%) in the sol–gel synthesis of silica glasses, either pure or doped with Ce and Er. In pure samples the Si–OH level is heavily reduced to ∼3 × 10−3 mol%. In doped samples the effect is more limited. The presence of residual fluorine, proved by the Si–F vibrational absorption at ∼950 cm−1, causes deep changes in the rare-earth (RE) crystal field spectra, which are characterized by narrow absorption lines. The lines broaden if the temperature is increased from 9 to 300 K. The result is discussed in terms either of local ordering around the rare earth ion or by its partial decoupling from the disordered host matrix, due to the creation of network terminating sites.