Crystal-field spectroscopy of Eu3+ doped silica glasses

Fourier transform absorption spectroscopy in the 500-6000 cm-1 and 9-300 K ranges is applied to monitor the effects produced by Eu3+ incorporation into sol-gel silica samples doped with concentration increasing from 0.001 to 10 mol%. The aim is to investigate the formation of aggregates by exploiting the Eu3+ crystal-field transitions. Complementary microreflectance and Raman spectra are also measured in the range of silica intrinsic vibrational modes to confirm the hypothesis of matrix modification induced by increasing doping levels. Evidences of clustering are found for high Eu3+ concentrations. Up to 3 mol% the crystal-field line intensities gradually increase and the OH- content smoothly decreases. A further increase to 10 mol% causes drastic, remarkable changes, i.e. sharp crystal-field lines appear which narrow by lowering the temperature. Furthermore, the OH- related bands are no longer detectable. For concentrations up to 3 mol% the aggregates are amorphous as the silica matrix, while for the Eu3+ 10 mol% sample they show a rather ordered structure.