Structural transformations in sodium silicate liquids under pressure: New static and dynamic structure analyses

The anomalous behavior displayed by silicate liquid/glass is a long-standing issue in physics, earth sciences, and glass technology. One such well-known unusual behavior is the contrary pressure dependence of silica-rich liquids on transport coefficients in comparison to other liquids. To investigate the origin of this anomalous behavior of silicate liquids, several analyses are newly developed and applied to molecular dynamics simulations of sodium silicate liquids. The cluster analyses performed on an Si-O network and simplex sphere reveals obvious structural changes arising as a function of pressure and composition. At lower pressure and/or low silica content, all simplex oxygen spheres containing the modifier cations are connected together, and multiple Si-O networks exist in the liquid structure. In contrast, at high pressure and/or high silica content, all SiO4 tetrahedra belong to a single cluster, and the cluster of simplex oxygen spheres containing the modifier cations is split into multiple clusters. The collective function, which is found to decrease with increasing pressure, reveals the pressure dependence of the dynamic structure. This change represents a change in the diffusive motion from large-group diffusion to small-group diffusion for the Si-O network. The complexity factor enables us to analyze the changes in the network structure. The decrease in the complexity factor of sodium disilicate liquids with increasing pressure represents an increase in the complexity of the Si-O network, mediated by the formation of additional connections with other domains via ring-opening reactions. These analyses reveal the pressure- and composition-dependent changes in the dominant network in silicate liquids. When the network of corner-sharing SiO4 tetrahedra is the dominant species in silicate liquids, the Si-O bonds become weaker as a result of the Si-O-Si angle bending, and the shear viscosity decreases with increasing pressure. In contrast, the transport coefficients show normal pressure dependence when the cation domain is fully connected in silicate liquids.