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.

中文翻译：

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在75 K观察到的亚纳米孔中H ₂的相变

在这里，我们报告了局部石墨形的Saran碳中吸附的H ₂的相变，其亚纳米孔的宽度为0.5-0.65 nm，其中两层氢只要紧密堆积就可以勉强挤压。在75 K时观察到相变，该温度远高于已知吸附剂会增加相变温度的其他系统：例如，H ₂在本体中以14 K熔化，但在石墨上以20 K熔化，因为固体H通过表面结构使图₂稳定。在这里，我们观察到在75 K和77–200 bar下的过渡：从低温低密度相到高温高密度相。我们将低密度相建模为主要由对H ₂组成的单层相称固体（基核自旋态，S = 0）和高密度相，是定向排列的双层相称固体，主要由邻位H ₂（S = 1）组成。我们将密度随温度的增加归因于长方形的邻位H ₂可以更密集地包装。使用两个实验观察到转变。高密度相与中子背向散射增加7.0±0.1倍有关。通常，氢不会产生反向散射（散射角> 90°）。这种反向散射与激励质量的不连续增加（从1.2 amu增加到21.0±2.3 amu）一起出现，这与定向有序相中的集体核自旋激发相关。使用氢吸附来测量膜密度。在对照活性炭材料中吸附的H _2中未观察到相变。