Atomistic modeling of multiple amorphous-amorphous transitions in SiO2 and GeO2 glasses at megabar pressures

We present molecular dynamics simulations of SiO2 and GeO2 glasses up to megabar pressures, 120 and 160 GPa, respectively, and show direct parallels between polyamorphism of glasses and polymorphism of their crystalline counterparts under compression. The glasses undergo a set of several smooth transformations in much the same manner as the corresponding crystals at nearly the same pressures, where coordination numbers of Si and Ge atoms considerably exceed 6, reaching 6.4 and 7.6, respectively, at maximum simulation pressures. The transformations in glasses, unlike those in crystals, occur with rather small hysteresis. High coordination states are not retained in a metastable form at room pressure