First-Principles\nStudy on Formation Mechanism of Six-Coordinated\nSi in Silicophosphate Glass

Six-coordinated Si (<sup>[6]</sup>Si) structures are\nreadily formed\nin silicophosphate glasses with high P<sub>2</sub>O<sub>5</sub> contents.\nAlthough experiments and simulations have provided some information\non the local configurations around <sup>[6]</sup>Si, further research\non the formation mechanism of <sup>[6]</sup>Si at the atomic scale\nis needed. To investigate the formation mechanism of <sup>[6]</sup>Si, we performed dynamic and static analyses based on first-principles\ncalculations. In first-principles molecular dynamics simulations with\nmodels in which all Si atoms are four-coordinated as the initial structure,\nwe observed that the coordination number of Si increased, and the\nP-Q<sup>2</sup> (P-Q<sup><i>n</i></sup>, where <i>n</i> represents the number of bridging oxygen atoms) changed to P-Q<sup>3</sup>. Atomic energy analysis revealed that the energy decreases\nwith the structural change from P-Q<sup>2</sup> to P-Q<sup>3</sup> exceeded the energy increase with an increase in the coordination\nnumber of Si, stabilizing of the entire system. We conclude that the\nkey factor in the formation of <sup>[6]</sup>Si is the decrease in\nenergy associated with the change from a nonbridging oxygen in a PO<sub>4</sub> tetrahedral structure to bridging oxygen.