Extended Data Fig. 1: Fitting of the SAXS data.

a, Comparison of experimental SAXS data of an initial GC sample before compression (red circles) in a DAC and the fitting results (solid and dashed lines). A spheroid form factor was used for the SAXS fitting, which assumes the pores to be spheres. This is one of the simplest shape models, and also a good estimation to the actual shape since a previous study found the pores in high-temperature treated GC approach a spherical shape (Ref. ¹⁵). The black dashed line represents the scattering contribution from pores by fitting to the Guinier region with the spherical shape model and the log-normal size distribution function. The pink dashed line represents Porod background scattering contribution from large size structures (e.g., beamline optics, diamond anvil windows, and large domains in GC). The green dashed line presents a low constant background possibly from air and/or other sources. It should be noted that the black dashed curve contains both Guinier and Porod regions of pore scattering. b, The size distribution of the pores in GC derived from the fitting in a. The average size of pores obtained from the SAXS data fitting is 1.5 ± 0.5 nm in diameter, which is close to the average grain size of argon (~3 nm) estimated from the XRD peak width (Fig. 3a). It should be noted that the pore size could vary to some extent through the complex compression-heating-cooling-decompression process to synthesize NDCs samples. Details regarding how and how much the pore size changes call for future efforts.