Tetrahedral-hexahedral-octahedral transition of SiO₂ glass at high pressure

The tetrahedral-octahedral phase transition in silica glass is of fundamental significance to high-pressure research due to its important role in geoscience and condensed matter physics. However, the formation and role of five-coordinate structural units within silica glass during this transformation process remain unclear. This paper presents a comprehensive study of ab initio molecular dynamics (AIMD) simulations of SiO₂ glass under high-pressure conditions. As the volume compression ratio (V/V₀) decreases to V/V₀ ≤ 0.59, a five-coordinate hexahedral SiO5 unit (denoted [5]Si) emerges suddenly, accounting for approximately 50% of the polyhedral units. This tetrahedral-to-hexahedral transition exhibits characteristics similar to a first-order phase transition. Within the compression ratio range of 0.5 ≤ V/V₀ ≤ 0.59, the [5]Si hexahedral structural unit becomes the dominant polyhedral species. Research has revealed that the [5]Si hexahedral unit plays a pivotal role in forming the [6]Si octahedral unit. Additionally, the tetrahedral-octahedral transition in SiO₂ glass under high pressure involves a tetrahedral-hexahedral-octahedral phase transformation whose mechanism is analogous to that of GeO₂ glass under extreme conditions. .