Structural investigation of sol–gel derived SiO₂–GeO₂ noncrystalline and nanocomposite materials

Noncrystalline and nanocomposite materials of (1−x)SiO₂·xGeO₂ system, with Si:Ge ratio from 8:1 to 2:1 (x = 0.111; 0.142; 0.2; 0.333), initially obtained by sol–gel method, were characterized by thermal analyses, X-ray diffraction, nuclear magnetic resonance and Raman spectroscopy. According to DTA and XRD results, the noncrystalline state of the as-prepared samples is stable up to 1,000 °C and only after 30 min heat treatment at 1,200 °C the samples become partial crystalline, due to development of cristobalite and quartz nanocrystals. Solid-state ²⁹Si MAS-NMR was employed in order to characterize the local structure around silicon as a function of composition and thermal history of the samples. The NMR data indicate the presence of Q², Q³ and Q⁴ units in all samples. The fraction of the highly interconnected SiO₄ tetrahedra increases both with germanium content and with annealing temperature. The Raman spectroscopy results evidence structural changes related to silicon- and germanium-oxygen units but also to their interconnection, that depend on Si:Ge ratio and annealing temperature.