Ranieri Umbertoluca, Di Cataldo Simone, Rescigno Maria, Monacelli Lorenzo, Gaal Richard, Santoro Mario, Andriambariarijaona Leon, Parisiades Paraskevas, De Michele Cristiano, Bove Livia Eleonora
Dipartimento di Fisica, Sapienza Università di Roma, 00185 Roma, Italy.
Centre for Science at Extreme Conditions and School of Physics and Astronomy, University of Edinburgh, EH9 3FD Edinburgh, United Kingdom.
Proc Natl Acad Sci U S A. 2023 Dec 26;120(52):e2312665120. doi: 10.1073/pnas.2312665120. Epub 2023 Dec 18.
Hydrogen hydrates are among the basic constituents of our solar system's outer planets, some of their moons, as well Neptune-like exo-planets. The details of their high-pressure phases and their thermodynamic conditions of formation and stability are fundamental information for establishing the presence of hydrogen hydrates in the interior of those celestial bodies, for example, against the presence of the pure components (water ice and molecular hydrogen). Here, we report a synthesis path and experimental observation, by X-ray diffraction and Raman spectroscopy measurements, of the most H[Formula: see text]-dense phase of hydrogen hydrate so far reported, namely the compound 3 (or C[Formula: see text]). The detailed characterisation of this hydrogen-filled ice, based on the crystal structure of cubic ice I (ice I[Formula: see text]), is performed by comparing the experimental observations with first-principles calculations based on density functional theory and the stochastic self-consistent harmonic approximation. We observe that the extreme (up to 90 GPa and likely beyond) pressure stability of this hydrate phase is due to the close-packed geometry of the hydrogen molecules caged in the ice I[Formula: see text] skeleton.
氢化水合物是我们太阳系外行星、其中一些卫星以及海王星类系外行星的基本组成成分之一。其高压相的细节以及形成和稳定的热力学条件是确定这些天体内部是否存在氢化水合物的基本信息，例如，与纯组分（水冰和分子氢）的存在情况相对比。在此，我们报告了一种合成路径，并通过X射线衍射和拉曼光谱测量进行了实验观察，发现了迄今为止报道的氢化水合物中氢密度最高的相，即化合物3（或C）。基于立方冰I（冰I）的晶体结构，通过将实验观察结果与基于密度泛函理论和随机自洽谐波近似的第一性原理计算进行比较，对这种充满氢的冰进行了详细表征。我们观察到，这种水合物相在极端压力（高达90吉帕甚至可能更高）下的稳定性是由于被囚禁在冰I骨架中的氢分子形成了密堆积几何结构。
