  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.
