Electrostatic force driven helium insertion into ammonia and water crystals under pressure

Abstract

Helium, ammonia and ice are among the major components of giant gas planets, and predictions of their chemical structures are therefore crucial in predicting planetary dynamics. In this work, we demonstrate a strong driving force originated from the alternation of the electrostatic interactions for helium to react with crystals of polar molecules such as ammonia and ice. Here we show ammonia and helium can form thermodynamically stable compounds above 45 GPa, while ice and helium can form thermodynamically stable compounds above 300 GPa. The changes in the electrostatic interactions provide the driving force for helium insertion under high pressure, but the mechanism is very different to those that occur in ammonia and ice. This work extends the reactivity of helium into new types of compounds and demonstrates the richness of the chemistry of this most stable element in the periodic table.