The influence of sodium oxide and temperature on the atomic structure and mechanical properties of silicate glasses: A molecular dynamics and Brillouin light scattering study

Abstract

This study investigates the structural and mechanical properties of sodium silicate glasses (Na₂O-SiO₂) based on Na₂O content (5–35 mol%) and temperature (300–600 K) using Brillouin Light Scattering (BLS) spectroscopy and Molecular Dynamics (MD) simulations. Key structural parameters are examined to understand how composition, temperature, and network structure interact. The results reveal significant network depolymerization as the Na₂O content increases, driven by Na-rich clusters that depend on the amount of Na₂O. Na diffusion is also studied and connected to its Voronoi volume. At low Na₂O, Na atoms are associated with larger Voronoi volumes, making diffusion easier due to thermal agitation and densification of the silicate network. At high Na₂O, Na atoms occupy smaller Voronoi volumes, requiring more energy to diffuse due to congestion of the Na sub-network and rigidification. Elastic constants ($C_{11}$, $C_{44}$) show different patterns: $C_{11}$ decreases below 20% Na₂O but increases at higher concentrations due to depolymerization and densification of the Na network, while $C_{44}$ steadily decreases, showing less resistance to shear due to the increasing presence of non-bridging oxygens (NBOs) (with only a minor contribution from free oxygens, FOs, whose concentration remains very low, $\sim$0.6%). Experimental results also show that at low Na₂O ($<$11%–12% for $C_{11}$, $<$15% for $C_{44}$), elastic constants increase with temperature due to local structural rearrangements and densification effects, while at higher Na₂O content, they decrease because of thermal softening driven by enhanced Na${}^{+}$ mobility and disruption of the silicate network. MD simulations confirm these trends with Na₂O content and the same trends at high Na₂O with temperature. However, at low Na₂O, the elastic constants decrease slightly with temperature. Young’s modulus, shear modulus, bulk modulus, and Poisson’s ratio are determined from $C_{11}$ and $C_{44}$. Novel metrics related to oxygen types and $Q_i$ (where $i$ is the number of bridging oxygens) are suggested to connect structural and mechanical findings, showing how they relate to either $C_{11}$ or $C_{44}$. The study highlights the competing mechanisms of thermal agitation, densification, depolymerization, congestion, and rigidification that collectively dictate the structural and mechanical behavior of sodium silicate glasses.