First-Principles Study on Formation Mechanism of Six-Coordinated Si in Silicophosphate Glass.

Six-coordinated Si (^[6]Si) structures are readily formed in silicophosphate glasses with high P₂O₅ contents. Although experiments and simulations have provided some information on the local configurations around ^[6]Si, further research on the formation mechanism of ^[6]Si at the atomic scale is needed. To investigate the formation mechanism of ^[6]Si, we performed dynamic and static analyses based on first-principles calculations. In first-principles molecular dynamics simulations with models in which all Si atoms are four-coordinated as the initial structure, we observed that the coordination number of Si increased, and the P-Q² (P-Qⁿ, where n represents the number of bridging oxygen atoms) changed to P-Q³. Atomic energy analysis revealed that the energy decreases with the structural change from P-Q² to P-Q³ exceeded the energy increase with an increase in the coordination number of Si, stabilizing of the entire system. We conclude that the key factor in the formation of ^[6]Si is the decrease in energy associated with the change from a nonbridging oxygen in a PO₄ tetrahedral structure to bridging oxygen.