Transparent glass-ceramics with a Li₂O–Al₂O₃–SiO₂ (LAS) system have been extensively utilized in optical systems in which thermal stability is of utmost importance. This study is aimed to develop thermal treatment routes that can effectively control the structure of transparent LAS glass-ceramics and tune its thermal expansion coefficient within a wide range for novel applications in photonics and integrated optics. The optimal conditions for the nucleation and crystallization of LAS glass were determined by means of differential scanning calorimetry and a polythermal analysis. XRD, Raman spectroscopy, and TEM microscopy were employed to examine the structural changes which occurred after heat treatments. It was found that the second stage of heat treatment promotes the formation of β-eucryptite-like solid solution nanocrystals, which enables effective control of the coefficient of thermal expansion of glass-ceramics in a wide temperature range of −120 to 500 °C. This work provides novel insights into structural rearrangement scenarios occurring in LAS glass, which are crucial for accurately predicting its crystallization behavior and ultimately achieving transparent glass-ceramics with desirable properties.

The glass crystallization regime plays a crucial role in the fabrication of glass ceramics: it affects both phase composition and microstructure, and thus the properties of the final product. In the search for new glass-ceramic materials, the development of a proper heat-treatment schedule involves the utilization of numerous glass samples that need to be thermally treated and then investigated to determine the values of the target characteristics. In this study, we evaluated the effect of crystallization temperature on the glass structure, phase composition, and hardness of glass ceramics in the ZnO-MgO-Al₂O₃-SiO₂ system containing TiO₂ and ZrO₂ as nucleators. To maximize the number of heat treatments, we performed polythermal crystallization of the glass in a wide temperature range with the help of a gradient furnace. Using X-ray diffraction, Raman spectroscopy, and transmission electron microscopy, we showed the precipitation of gahnite nanocrystals as the main phase in the bulk of a single glass sample and observed a gradual change in its microstructure, transparency, and hardness. The dependence of Vickers hardness values on heat treatment temperature was found to follow a non-linear trend, revealing the optimal thermal range for glass crystallization.