Figure 1
(A) Pressure and temperature trajectories for two decompositions of HDPE relative to the carbon phase diagram13, 14. HDPE was initially compressed to either 11.5 GPa (‘a’ series, red) or 29.0 GPa (‘b’ series, blue) before heating to decomposition at T > 2500 K for ~ 1 s. Following decomposition, ambient T XRD and SAXS measurements were performed over a series of pressures, denoted by circles a1-a5 (red) and b1-b2 (blue). Horizontal dashed lines indicate the ambient T phase boundaries for solid methane crystal structures I, A, B, and HP1. (B) Representative XRD patterns showing peaks corresponding to diamond and hydrocarbon products after heating HDPE to > 2500 K at 11.5 GPa (red, a1) and after heating to > 2500 K at 29.0 GPa and subsequent pressure release to 7.5 GPa (blue, b2). The two high intensity peaks correspond to the Re gasket and stars denote peaks corresponding to ruby. For reference, positions of HDPE (110) and (200) peaks at ambient conditions (solid lines) and at P = 15 GPa, T = 300 K (dashed lines) are at lower q than the peaks assigned to hydrocarbon products15.
