Phase Transition of H₂ in Subnanometer Pores Observed at 75 K

Here we report a phase transition in H₂ adsorbed in a locally graphitic Saran carbon with subnanometer pores 0.5-0.65 nm in width, in which two layers of hydrogen can just barely squeeze, provided they pack tightly. The phase transition is observed at 75 K, temperatures far higher than other systems in which an adsorbent is known to increase phase transition temperatures: for instance, H₂ melts at 14 K in the bulk, but at 20 K on graphite because the solid H₂ is stabilized by the surface structure. Here we observe a transition at 75 K and 77-200 bar: from a low-temperature, low-density phase to a high-temperature, higher density phase. We model the low-density phase as a monolayer commensurate solid composed mostly of para-H₂ (the ground nuclear spin state, S = 0) and the high-density phase as an orientationally ordered bilayer commensurate solid composed mostly of ortho-H₂ (S = 1). We attribute the increase in density with temperature to the fact that the oblong ortho-H₂ can pack more densely. The transition is observed using two experiments. The high-density phase is associated with an increase in neutron backscatter by a factor of 7.0 ± 0.1. Normally, hydrogen produces no backscatter (scattering angle >90°). This backscatter appears along with a discontinuous increase in the excitation mass from 1.2 amu to 21.0 ± 2.3 amu, which we associate with collective nuclear spin excitations in the orientationally ordered phase. Film densities were measured using hydrogen adsorption. No phase transition was observed in H₂ adsorbed in control activated carbon materials.