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Glass ceramics based on Li 2 O/Al 2 O 3 /SiO 2  are of high economic importance, as they often show very low coefficients of thermal expansion. This enables a number of challenging applications, such as cooktop panels, furnace windows or telescope mirror blanks. Usually, the crystallization of the desired LAS crystal phases within the glasses must be tailored by a careful choice of crystallization schedule and type of nucleation agents to be used. The present work describes the formation of nanocrystalline TiO 2  within an LAS base composition that contains solely TiO 2  as nucleating agent. Using a combination of scanning transmission electron microscopy as well as X-ray absorption spectroscopy, it is found that a mixture of four- and six-fold coordinated Ti 4+  ions exists already within the glass. Heating of the glass to 740 °C immediately changes this ratio towards a high content of six-fold coordinated Ti, which accumulates in liquid-liquid phase-separation droplets. During the course of thermal treatment, these droplets eventually evolve into nanocrystalline TiO 2  precipitations, in which Ti 4+  is six-fold coordinated. Thus, it is shown that the nucleation of nanocrystalline TiO 2  is initiated by a gradual re-arrangement of the Ti ions in the amorphous, glassy matrix, from a four-fold towards a six-fold coordination.

The effect of TiO2 on nucleation and crystallization of a Li2O-Al2O3-SiO2 glass investigated by XANES and STEM