Figure 4: Summary of the experimental results.

(a) Recorded pressure–density diagram for compressed pyrolytic graphite (ρ₀=2.21 g cm⁻³) compared with literature data without structure information and a first principles phase diagram³³. At lower pressures, there is very good agreement, whereas at higher pressures, due to the formation of lonsdaleite, we observe higher densities than predicted by a proposed shock Hugoniot³¹, which instead suggests a transition to the liquid. (b) Recorded pressure–density diagram for porous polycrystalline graphite (ρ₀=1.84 g cm⁻³) compared with experiments without structure information^16,32 and a first principles phase diagram. Comparable to the pyrolytic samples, the diamond formation is not fully completed within few nanoseconds at lower pressures, resulting in a broad peak which sharpens up to ∼100 GPa. At higher pressures, however, the increasing temperature leads to melting of the diamond structure, resulting in broader and fainter diffraction peaks in agreement with a bonded liquid¹⁸. No signature of lonsdaleite is observed when compressing porous graphite.