Tuning fragility in sodium lead borate glasses: Unveiling the interplay between compositions, Stokes–Einstein breakdown, and dynamical heterogeneity

Abstract

Viscosity is a critical determinant of a liquid’s ability to form a glass upon cooling from a high-temperature state. As glass-forming liquids are cooled, their viscosity, or equivalently the structural relaxation time, increases rapidly near the glass transition temperature, a universal hallmark of glasses. The temperature dependence of viscosity is characterized by fragility, which varies widely among glassy liquids: some show the Arrhenius temperature dependence of viscosity or relaxation time (“strong” liquids), while others exhibit super-Arrhenius behavior (“fragile” liquids). We performed extensive molecular dynamics simulations on a realistic glass-forming system, sodium–lead–borate (Na2O–PbO–B2O3), to investigate how increasing lead oxide (PbO) content at the expense of boron oxide (B2O3) influences viscosity and fragility. Our results show a transition from strong to fragile behavior with increasing PbO concentration, elucidating the role of chemical composition in driving this transition. We further examine the Stokes–Einstein (SE) relation, the Kohlrausch–Williams–Watts (KWW) stretch exponent (βkww), and dynamical heterogeneity across the strong-to-fragile spectrum. We find that SE violation becomes more pronounced with increasing fragility, while βkww decreases, indicating stronger deviation from exponential relaxation in fragile glasses. Interestingly, dynamical heterogeneity, characterized by the four-point susceptibility [χ4(t)] and the non-Gaussian parameter [α2(t)], is slightly enhanced in strong glasses despite weaker SE breakdown and higher βkww values. Furthermore, our results suggest that different local structural units play distinct roles in shaping dynamical heterogeneity in strong and fragile glasses. These findings underscore the intricate interplay between fragility, the Stokes–Einstein relation, and dynamical heterogeneity, while emphasizing the crucial role of glass composition in tuning viscosity in real glass-forming systems.