Characterizing medium-range order structure of binary silicate glasses using ring analysis and persistent homology

Several fundamental questions about the medium-range order (MRO) structure of oxide glasses remain unanswered. How do we define MRO in glass? Should we only consider the covalently bonded rings or also repeating patterns of non-chemically bonded atom clusters? Is the first sharp diffraction peak (FSDP) in the structure factor only constituted by those rings? In this study, by focusing on binary silicate glasses, we compare the MRO structure as determined using persistent homology and classical ring analysis. While the latter only identifies chemically bonded rings, the former captures both chemically and non-chemically bonded ring/loop structures. Our analyses are based on atomic configurations established through classical molecular dynamics simulations of three series of alkali silicate glasses with varying modifier content. First, we characterize the size and shape of chemically bonded rings using persistent homology and study how they contribute to the FSDP. We also show that the covalently bonded loops can be directly extracted using persistent homology by ignoring the modifiers from the analysis and setting the initial radii for both Si and O atoms to zero. Then, we demonstrate that although the chemically bonded rings contribute to the FSDP, especially at low modifier content, nonbonded MRO features also need to be considered to fully explain the FSDP.