Computational phase diagrams of noble gas hydrates under pressure

We present results from a first-principles study on the stability of noble gas-water compounds in the pressure range 0-100 kbar. Filled-ice structures based on the host water networks ice-Ih, ice-Ic, ice-II, and C0 interacting with guest species He, Ne, and Ar are investigated, using density functional theory (DFT) with four different exchange-correlation functionals that include dispersion effects to various degrees: the non-local density-based optPBE-van derWaals (vdW) and rPW86-vdW2 functionals, the semi-empirical D2 atom pair correction, and the semi-local PBE functional. In the He-water system, the sequence of stable phases closely matches that seen in the hydrogen hydrates, a guest species of comparable size. In the Ne-water system, we predict a novel hydrate structure based on the C0 water network to be stable or at least competitive at relatively low pressure. In the Ar-water system, as expected, no filled-ice phases are stable; however, a partially occupied Ar-C0 hydrate structure is metastable with respect to the constituents. The ability of the different DFT functionals to describe the weak host-guest interactions is analysed and compared to coupled cluster results on gas phase systems.

本研究针对压力区间0~100 kbar内稀有气体-水合物（noble gas-water compounds）的稳定性开展第一性原理（first-principles）研究，并汇报相关计算结果。本研究以氦（He）、氖（Ne）、氩（Ar）为客体物种（guest species），针对基于冰-Ih、冰-Ic、冰-II及C0主体水骨架（host water networks）的填充冰结构（filled-ice structures）开展系统研究，采用包含不同程度色散效应（dispersion effects）的四类交换关联泛函（exchange-correlation functionals）：非局域密度型optPBE-范德华（optPBE-van derWaals, vdW）与rPW86-vdW2泛函、半经验D2原子对修正方法，以及半局域PBE泛函。在氦-水体系中，稳定物相的演化序列与尺寸相近的氢水合物（hydrogen hydrates）体系中的物相序列高度吻合。在氖-水体系中，本研究预测基于C0水骨架的新型水合物结构在相对低压下可稳定存在，或至少具备热力学竞争力。在氩-水体系中，正如预期，无任何填充冰相可稳定存在；但部分占据的氩-C0水合物结构相对于其组分相而言为亚稳态（metastable）。本研究分析了不同DFT泛函描述弱主体-客体相互作用的能力，并将其与气相体系的耦合簇（coupled cluster）计算结果进行对比。