Abstract

Helium and methane are major components of giant icy planets and are abundant in the universe. However, helium is the most inert element in the periodic table and methane is one of the most hydrophobic molecules, thus whether they can react with each other is of fundamental importance. Here, our crystal structure searches and first-principles calculations predict that a He₃CH₄ compound is stable over a wide range of pressures from 55 to 155 GPa and a HeCH₄ compound becomes stable around 105 GPa. As nice examples of pure van der Waals crystals, the insertion of helium atoms changes the original packing of pure methane molecules and also largely hinders the polymerization of methane at higher pressures. After analyzing the diffusive properties during the melting of He₃CH₄ at high pressure and high temperature, in addition to a plastic methane phase, we have discovered an unusual phase which exhibits coexistence of diffusive helium and plastic methane. In addition, the range of the diffusive behavior within the helium-methane phase diagram is found to be much narrower compared to that of previously predicted helium-water compounds. This may be due to the weaker van der Waals interactions between methane molecules compared to those in helium-water compounds, and that the helium-methane compound melts more easily.

中文翻译：

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氦甲烷化合物中塑性相和部分扩散相的共存

 抽象的

氦和甲烷是巨大冰行星的主要成分，在宇宙中含量丰富。然而，氦是元素周期表中最惰性的元素，而甲烷是最疏水的分子之一，因此它们是否可以相互反应至关重要。在这里，我们的晶体结构搜索和第一性原理计算预测He ₃ CH ₄化合物在55至155 GPa的宽压力范围内是稳定的，而HeCH ₄化合物在105 GPa左右变得稳定。作为纯范德华晶体的一个很好的例子，氦原子的插入改变了纯甲烷分子的原始堆积，并且在很大程度上阻碍了甲烷在较高压力下的聚合。通过分析He ₃ CH ₄在高压高温下熔化过程中的扩散特性，除了塑性甲烷相之外，我们还发现了一种不寻常的相，它表现出扩散氦和塑性甲烷的共存。此外，与之前预测的氦-水化合物相比，氦-甲烷相图中的扩散行为范围要窄得多。这可能是由于与氦-水化合物相比，甲烷分子之间的范德华相互作用较弱，并且氦-甲烷化合物更容易熔化。