A modified random network model for P₂O₅-Na₂O-Al₂O₃-SiO₂ glass studied by molecular dynamics simulations

We investigated the short- and medium-range structural features of sodium aluminosilicate glasses with various P₂O₅ (0-7 mol%) content and Al/Na ratios ranging from 0.667 to 2.000 by using molecular dynamics simulations. The local environment evolution of network former cations (Si, Al, P) and the extent of clustering behavior of modifiers (Na⁺) is determined through pair distribution function (PDF), total correlation function (TDF), coordination number (CN), Q _x distribution and oxygen speciation analysis. We show that Al-O-P and Si-O-Al linkage is preferred over other connections as compared to a random model and that Si-O-Si linkage is promoted by the P₂O₅ addition, which is related to structural heterogeneity and generates well-separated silicon-rich and aluminum-phosphorus-rich regions. Meanwhile, due to the relatively high propensity of Al to both Si and P, heterogeneity can be partly overcome with high Al content. A small amount of Si-O-P linkages have been detected at the interface of separated regions. Clustering of Na⁺ is also observed and intensified with the addition of P₂O₅. Based on the simulated structural information, a modified random network model for P₂O₅-bearing sodium aluminosilicate glass has been proposed, which could be useful to optimize the mobility of sodium ions and design novel functional glass compositions.