Structural Mechanism for Gas Permeability in Noncrystalline Carbon under Pressure

Volatiles can form novel compounds and exhibit extraordinary properties under high pressure. The recently developed nanostructured diamond capsules (NDCs) enable the preservation of otherwise typically unquenchable high-pressure volatiles for fundamental research and practical applications in ambient environments. A critical process in NDC synthesis is the pressure-driven diffusion of volatiles into the enclosed nanopores of glassy carbon, the precursor material. However, the structural pathways that allow such diffusion remain unclear, as glassy carbon is typically considered to be extremely impermeable. Here, we combined in situ high-pressure synchrotron X-ray diffraction and small-angle X-ray scattering, complemented by transmission electron microscopy, to investigate the pressure-induced diffusion of two model gases, helium and argon, into glassy carbon and amorphous carbon. Our findings reveal that gas penetration occurs through disordered structural defects in glassy carbon rather than via the usually expected interlayer spacing between its weakly bonded graphene-like layers. Based on this mechanism, we further demonstrate that moderate pressures can also drive gas diffusion into various amorphous carbon nanospheres, identifying them as viable alternative precursors for NDCs. These findings close a critical gap in understanding gas permeability in disordered carbon under pressure and provide essential guidance for tailoring carbon precursors to optimize NDCs for applications with high-pressure volatiles.