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

Abstract

We investigate the structure, phase separation, glass transition, and crystallization in a mixed network former glass series, i.e., B₂O₃–Al₂O₃–SiO₂–P₂O₅ glasses with varying SiO₂/B₂O₃ molar ratio. All the studied glasses exhibit two separate glassy phases: droplet phase (G₁) with the size of 50–100 nm and matrix phase (G₂), 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 B₂O₃ for SiO₂, but the magnitude of the decrease is larger for T_g1. Based on nuclear magnetic resonance and Raman spectroscopy results, we infer that the G₁ phase is rich in boroxol rings, while the G₂ phase mainly involves the B–O–Si network. Both phases contain BPO₄- and AlPO₄-like units. Ordered domains occur in G₂ 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 G₂. These findings are useful for understanding glass formation and phase separation in mixed network former oxide systems, and for tailoring their properties.