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

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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.