Nano-phase separation and structural ordering in silica-rich mixed network former glasses

We investigate the structure, phase separation, glass transition, and crystallization in a mixed network former glass series, i.e., B ₂O ₃-Al ₂O ₃-SiO ₂-P ₂O ₅ glasses with varying SiO ₂/B ₂O ₃ molar ratio. All the studied glasses exhibit two separate glassy phases: droplet phase (G ₁) with the size of 50-100 nm and matrix phase (G ₂), corresponding to a lower calorimetric glass transition temperature (T _g1) and a higher one (T _g2), respectively. Both T _g values decrease linearly with the substitution of B ₂O ₃ for SiO ₂, but the magnitude of the decrease is larger for T _g1. Based on nuclear magnetic resonance and Raman spectroscopy results, we infer that the G ₁ phase is rich in boroxol rings, while the G ₂ phase mainly involves the B-O-Si network. Both phases contain BPO ₄- and AlPO ₄-like units. Ordered domains occur in G ₂ upon isothermal and dynamic heating, driven by the structural heterogeneity in the as-prepared glasses. The structural ordering lowers the activation energy of crystal growth, thus promoting partial crystallization of G ₂. These findings are useful for understanding glass formation and phase separation in mixed network former oxide systems, and for tailoring their properties.