Fig. 5.

Dependence of OPF on A–O CN for SiO₂ glass from this study and GeO₂ glass from ref. 16. We compare experimental data to calculated values for the Fe₂P-type and cotunnite structural polymorphs for SiO₂ (red left/right triangles) (30) and GeO₂ (white up/down triangles) (32). Hereby, different directions of the symbols are for different cutoff values (2.1 and 2.5 Å) for the O–O MEFIR distance calculations for these crystal structures. The dashed blue line represents the Kepler conjecture (KC) marking the densest possible OPF [$\sim$0.74 (34)]. The black diamond represents an “ideal” hypothetical close-packed AO₂ structure where both atoms have the same size and contribute to the close packing equally; thus, A–B coordination will be 12, but the OPF ${\eta }_N$ is only two-thirds of the KC. The shaded areas are linear extrapolations from the ideal AO₂ structure through the values of predicted crystalline high pressure phases to sixfold coordination. The magenta crosses show the results of the ab initio MD (AIMD) simulation for different densities (7–16 g/cm³ at 4,000 K).