Phase separation in gel-derived materials, separation and crystallization of SnO₂ within an amorphous SiO₂ matrix

On heating a gel of composition 33.5SnO₂−66.5SiO₂ (wt. %) up to 1050 °C, SnO₂ crystallization occurred with different mechanisms according to thermal treatments. Thermal analysis, SAXS (Small Angle X-Ray Scattering), WAXS (Wide Angle X-Ray Scattering), and TEM (Transmission Electron Microscopy) results obtained for samples treated at different temperatures demonstrated that the SnO₂ load is divided into two moieties, one composed of isolated SnO₂ particles suitable for primary crystallization yielding crystallites with 4.0 nm of average size; the other, being dissolved in SiO₂, remains in the amorphous SiO₂/SnO₂ solid solution up to the highest temperature. The presence of these two phases accounts for SnO₂ surface segregation at the expense of the SnO₂ concentration of neighboring outer layers and the independence of apparent density on temperature.