Structural fluctuations and role of Ti as nucleating agent in an aluminosilicate glass

Nucleation is the initiating event for the formation of any crystal from undercooled melt. Although the classical nucleation theory describes qualitatively many aspects of the crystallization process, the mechanisms favoring the first steps of nucleation are still unknown and required the full identification of the catalytic sites. In this work, we report a structural investigation of the system 2MgO–2Al ₂O ₃–5SiO ₂ + TiO ₂ , a major glass–ceramic for scientific and industrial interest, in order to understand the structural influence of Ti as a nucleating agent. X-ray scattering and ²⁷Al Nuclear Magnetic Resonance studies were carried out on 2MgO–2Al ₂O ₃–5SiO ₂ + xTiO ₂ glasses, with 0 mol% ≤ x ≤ 15.5 mol%. No evidence of phase separation can be detected in the initial glasses by Transmission Electron Microscopy at nanoscale and Ti atoms appear to be homogeneously distributed within the glassy structure. We explain that the presence of TiO ₂ favors nucleation by the formation in the initial glass of high-coordinated Al species and the presence of structural fluctuations that mimic the initial crystalline phase precipitating in the glass. The understanding of the parent glass structure appears as a critical constraint to understand the pathways promoting nucleation.