Evidence of Hydrogen Intercalation in Graphite under High Pressure: A Comparative X-Ray Study with Helium

X-ray diffraction studies of graphite–hydrogen mixtures were performed under pressures up to 50 GPa, along with comparative measurements of graphite–helium mixtures up to 60 GPa. Evidence of hydrogen intercalation in graphite was observed beginning near 5.5 GPa, indicated by an anomalous expansion of the c-axis lattice parameter—opposite to the expected monotonic compression under pressure. This pressure corresponds to the solidification of hydrogen at room temperature, which likely facilitates intercalation through a significant reduction in its kinetic diameter, allowing diffusion into graphite interlayers. In contrast, the graphite–helium mixtures exhibited no such anomaly near 5.5 GPa; instead, the c-axis decreased continuously with pressure, consistent with normal lattice compression. The distinct pressure-dependent behaviors of graphite in hydrogen and helium environments highlight the potential role of hydrogen intercalation. Further studies, such as neutron diffraction, are planned to confirm and elucidate the intercalation mechanism.