Non-local Chemistry Driven by Cation-Anion Size Disparity in Helium Inserted Compounds under High Pressure

Contrary to the theory that He cannot be inserted into AB-type ionic compounds due to an increase in Madelung energy, our crystal structure search and first-principles calculations show that He can form stable compounds with sodium halides (NaX, X = Cl, Br, I) under high pressure. Unlike He insertion reactions in A₂B-type compounds, which are driven by cation-anion charge disparities, these new reactions are influenced by non-local chemistry caused by the cation-anion size disparity. The significant size difference between Na⁺ and X⁻ enables structures that can accommodate He insertions through volume and interatomic distance disproportionation. Additionally, this size disparity creates substantial electrostatic repulsions between Na⁺ cations under pressure as the volumes of NaX compounds are reduced. The insertion of He atoms relieves these electrostatic repulsions, allowing a reduction in volume under high pressure without increasing Madelung energy. These findings expand the understanding of He reactivity, revealing a new chemistry governed by long-range electrostatic interactions rather than local chemical bonds.