Evidences of Rare Earth Ion Aggregates in a Sol-Gel Silica Matrix: The Case of Cerium and Gadolinium

Structural and morphological studies were carried out on cerium- and gadolinium-doped sol-gel silica glasses intended for scintillator applications, to deepen the understanding of rare earth ion incorporation into the glass matrix. Several compositions, ranging from 0 to 5 mol % Ce and from 0 to 8 mol % Gd, were studied by Raman spectroscopy. The vibrational response was compared to that of pure silica glasses: for cerium doping higher than 0.5 mol %, the F2g Raman mode, characteristic of CeO2, was observed. The presence of CeO2 nanocrystalline clusters, whose size depends on cerium concentration and thermal treatment, was confirmed also by X-ray powder diffraction (XRD) patterns and transmission electron microscopy (TEM) analyses. On the contrary, gadolinium-doped sol-gel silica glasses exhibited Raman spectra similar to those of pure silica glasses, at least for the investigated concentrations up to 8 mol %, and no crystalline particles were detected within the amorphous matrix.