Formation ofNH3−Xecompound at the extreme condition of planetary interiors

Abstract

Noble gas elements have been illustrated to exhibit chemical activity to form unconventional compounds at high pressure. In this paper, we report combined structure prediction and first-principles calculations to propose an unexpected stoichiometry of ${({\mathrm{NH}}_{3})}_{2}\mathrm{Xe}$ that becomes energetically stable >11 GPa. Here, ${\mathrm{NH}}_{3}$ in the compound remains in its molecular form up to at least 300 GPa, indicating that the incorporation of Xe could suppress the ionization of ${\mathrm{NH}}_{3}$. Ab initio molecular dynamics simulations reveal that ${({\mathrm{NH}}_{3})}_{2}\mathrm{Xe}$ transforms from a solid to a superionic, and finally to a fluid as the temperature increases. The superionic phase remains stable in the pressure and temperature region that covers the extreme conditions of the layer outside the core of planets such as Uranus, Neptune, Venus, and Earth. This suggests that ${({\mathrm{NH}}_{3})}_{2}\mathrm{Xe}$ is a possible constituent of planetary interiors. The current results provide theoretical evidence that Xe could be trapped inside planets during their evolution and could help to update models of planetary interiors.