Crystallization mechanism in sol-gel oxyfluoride glass-ceramics

The crystallization mechanism of LaF3 in sol-gel oxyfluoride glass-ceramics in the system SiO2-LaF3 has been studied using DTA, XRD, FTIR, HRTEM and NMR. Based on classical calculations of non-isothermal DTA curves involving Ozawas's plots, a diffusion-controlled process should be responsible for the fluorides crystallization similarly to that obtained for melt-quenching oxyfluoride glass-ceramics. Therefore, an increase of the crystal size and fraction is expected when increasing the time and/or temperature of the treatment. Nevertheless, XRD, FIR, HRTEM and NMR demonstrated that neither bigger crystals nor higher crystal fraction are obtained for treatment times longer than 1 min at 550 degrees C. However, further heat treatment causes that the structural order of the crystals is lost. On the other hand, typical fluorides crystallization temperatures (similar to 300 degrees C) in sol-gel materials are much lower than T-g of the matrix (similar to 1130 degrees C) thus being very different to what observed for oxyfluoride glasses prepared by melt-quenching. The proposed crystallization mechanism is a fast process that occurs when crystallization temperature is reached. This behavior is explained as a chemical reaction from the xerogels, followed by the fast precipitation of the nanocrystals without energy barrier. The nanocrystals are unstable in the surrounding SiO2 matrix for aging at crystallization or higher temperatures. These treatments lead to a dissolution process, favored by the compositional gradient between matrix and crystals, which drive the material to reach the equilibrium.