Tetrahedral-hexahedral-octahedral transition of GeO2 glass at high pressure

Abstract Pressure-induced tetrahedral-octahedral transitions have been one of the central topics in high-pressure research due to the importance of SiO2 and GeO2 glasses in condensed matter physics and geosciences. However, the existence and role of the fivefold coordinated structural unit remains elusive. While accurate determination of the different polyhedral structures in the glasses in high-pressure experiments is a formidable challenge, we have performed a comprehensive ab initio molecular dynamics (AIMD) simulation and the results agree well with the local structure determined by HP-XAFS experiments. With increasing pressure, the fivefold-coordinated hexahedral GeO5 unit ([5]Ge) suddenly appears with up to 40% polyhedrons, while the sixfold-coordinated GeO6 unit ([6]Ge) is negligible. The hexahedral [5]Ge become the dominant polyhedral species in 15-25 GPa. The results indicate that hexahedral [5]Ge plays an important role in the formation of octahedral [6]Ge and that the commonly considered tetrahedral-octahedral transition of GeO2 glass at high pressure is actually a tetrahedral-hexahedral-octahedral transition.