Crystal structures and properties of the CH4H2 compound under high pressure

Abstract

The structural, electronic, and dynamical properties and intermolecular interactions of CH₄H₂ are investigated based on first-principles calculations. Enthalpy calculations indicate that the P structure is the most stable phase below 15.6 GPa. On compression, the P2₁2₁2₁ phase possesses the lowest enthalpy, then the P2₁/C becomes energetically favorable above 98.2 GPa. The pressure-induced hardening behavior of H₂-bonds in the CH₄H₂ system reproduces the experimental data well. In addition, the CH₄ molecules remain as tetrahedra due to the weak interactions of molecular H₂ and CH₄. At higher pressure, the orientation of H₂ molecules in CH₄H₂ is also disordered and this resembles the findings for pure H₂. It shows that comparing to pure H₂, the addition of CH₄ does not have much limitations on the rotation of the H₂ molecules.