Structural characteristics and elasticities of coesite and coesite-II at high pressure

Silica, SiO₂, has attracted significant interest as one of the most important compounds in the fields of condensed-matter physics, materials science, and geoscience etc. Here, we theoretically investigate crystal structural characteristics and elastic properties of coesite and its high-pressure polymorph coesite-II at high pressures, which is critical to improve the understanding of densification mechanism of silica at high pressure. The driving mechanism for the pressure-induced transition pathways of coesite comes from both structural features and elastic effect. The phase transition of coesite to cosite-II is triggered by the shortening of Si1–O1 bond distance, and accompanied by elastic instability from a combination of softening elastic constants C ₄₄ and C ₄₆. The structural modulation of coesite-II along the b-axis direction, as proposed experimentally, results from, on the one hand, the bending of Si2–O15–Si3 angles and shortening of Si5–O4 and Si7–O4 distance with increasing pressure and, on the other hand, elastic softening associated with phonon instability. These results provide key insights into the densification mechanism of silica under high pressure. (paper)