Chemistry without Chemical Bonds: Reactivity of He with Ionic Compounds under High Pressure

Since its discovery 150 years ago and until very recently, Helium has remained the last naturally occurring element that does not form a stable solid compound. We propose and demonstrate, using structure prediction coupled with first principles calculations, that there is a general driving force for Helium to react with ionic compounds that contain an unequal number of cations and anions. The corresponding reaction products contain cations, anions, and helium, and are stabilized not by any local chemical bonds but by the long-range Coulomb interactions that are significantly modified by the insertion of helium atoms, especially under high pressure. We study the stability of the mixtures of He and MgF2, MgO, Li2O, LiF, CaF2, Na with density functional theory. As is presumed, MgF2, Li2O, CaF2 and Na, which have different numbers of anion and cation, can form stable structures with He while their counterparts, MgO and LiF, show a positive formation enthalpy when mixed with Helium. We calculate the internal and Madelung energy difference of formation of the compounds and they show similar behavior at high pressure. It is concluded that the Madelung energy decrease when He mixes with A2B or AB2 compounds resulting in the stability of AB2He or A2BHe compounds at high pressure.