Nanocrystalline apatite formation on bioactive glass in a sol–gel synthesis

• The sol–gel process facilitates the crystallization of apatite nanocrystals below Tg.
• Apatite nanocrystals are formed due to the segregation of Ca2 + and PO43 −.
• Silicate crystallization and phosphate crystallization follow distinct devitrification processes.

The goal of this study was to understand the process involved in the formation of a nanocrystalline apatite phase during bioglass formation via a sol–gel process. X-ray diffraction, Rietveld refinement and transmission electron microscopy were employed to evaluate the structural changes and ordering processes that occur during glass formation. The presence of nanocrystalline apatite domains was observed after thermal treatment at temperatures lower than Tg. The formation of this phase was directly related to the initial segregation of calcium nitrate and triethylphosphate from the amorphous silicate clusters during the drying process. After thermal treatment at 300 °C, the calcium nitrate decomposed, and calcite formed. Calcite was further decarbonized, and the remaining phosphate groups reacted with calcium, increasing the quantity of nanocrystalline apatite domains. Higher than Tg, these domains acquire a higher crystallinity that was easily identified among the other crystallization products, such as combeite and β-cristobalite, following independent events. Therefore, the sol–gel process generates suitable conditions for apatite crystallization even during the initial formation of a bioglass at low temperatures and without being in contact with biological fluids.