A High-Density Supercritical Fluid of H2

Abstract This study aimed to elucidate the structural evolution of supercritical fluid hydrogen (SCF H2) under high pressure. X-ray diffraction was performed on SCF H2 using synchrotron radiation at room temperature. The structure factor S(Q) was observed in a low-Q region at each pressure, revealing a crossover at around 0.56 GPa, consistent with our previous Raman spectroscopy results. The average volume per molecule was calculated at each pressure using the center-to-center distance between the nearest neighboring H2 molecules, reflecting van der Waals interactions and compression of the internuclear distance. The volumes calculated from the face-centered cubic model were consistent with those derived from the cited equation of state, with a slight discrepancy that increased gradually with the pressure. This finding indicates that further densification following the crossover occurs primarily by intra-atomic/molecular mechanisms rather than intermolecular shortening of distances, which cannot be adequately explained by the close-packed coordination model alone.