Xe anions in stable Mg-Xe compounds: the mechanism of missing Xe in earth atmosphere

TL;DR

This study uses first principles calculations to show that Xe and Kr can form stable negatively charged compounds with Mg under high pressure, explaining the missing Xe in Earth's atmosphere.

Contribution

It reveals that noble gases like Xe can form stable anionic compounds with Mg under high pressure, a novel mechanism for noble gas capture in Earth's interior.

Findings

MgXe and MgKr are stable intermetallic compounds at high pressure.

Xe can form negatively charged species in Earth's interior.

Temperature significantly stabilizes these compounds.

Abstract

The reactivity of noble gas elements is important for both fundamental chemistry and geological science. The discovery of the oxidation of Xe extended the doctrinal boundary of chemistry that a complete shell is inert to reaction. The oxidations of Xe by various geological substances have been researched in order to explain the missing Xe in earth atmosphere. Among many proposals, the chemistry mechanisms are straightforward as they identify chemical processes that can capture Xe in earth interior. However, all the mechanisms based on current noble gas chemistry face the same difficulty: the earth lower mantle and core are rich in metals and therefore their chemical environment is reductive. On the other hand, up till now, the opposite chemical inclination, the reductive propensity, i.e. gaining electrons and forming anions, has not been proposed and examined for noble gas elements. In this work, we demonstrate, using first principles calculations and an efficient structure prediction method, that Xe and Kr can form stable MgXe and MgKr compounds under high pressure. These compounds are intermetallic and Xe or Kr is negatively charged. We also find that elevated temperature has large effect in stabilizing MgXe and MgKr compounds. Our results show that the earth has the capability of capturing Xe but not Kr, which is consistent to the depletion of Xe in earth atmosphere. The stability of these compounds suggests that chemical species with completely filled shell may still gain more electrons filling their outer shell in chemical reactions.