Structural characterization of amorphous and nanostructured bismuth silicate xerogels

Bismuth silicate xBi2O3·(1 − x)SiO2 xerogels, 0 ≤ x ≤ 0.5, doped with gadolinium have been synthesized by sol–gel method. The study aims to evidence by X-ray diffraction (XRD), electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) the structural changes in amorphous Bi2O3–SiO2 xerogels, function of Bi2O3 content and heat treatment. As was proved by XRD, nanostructured samples were obtained after 30 min heat treatment at 400 °C of the as-prepared samples with x > 0, in the mainly amorphous matrices nanocrystals of Bi5.6Si0.5O9.4 phase being developed. The dimension of the nanocrystallites are function of the bismuth content, and vary from few nanometers to 25 nm for samples with x = 0.14 and x = 0.5 respectively. The local order around silicon, reflected by 29Si MAS NMR spectra, is less dependent on both composition and thermal history of the samples, showing that an amorphous silicon rich phase is present in all samples. The changes in the local order around Gd3+ ions, as reflected by the EPR spectra dependence on composition and especially on the heat treatment, support the assumption that a part of these paramagnetic ions is incorporated in the bismuth rich phase.