  Electrides are an emerging class of materials with highly-localized electrons
in the interstices of a crystal that behave as anions. The presence of these
unusual interstitial quasi-atom (ISQ) electrons leads to interesting physical
and chemical properties, and wide potential applications for this new class of
materials. Crystal defects often have a crucial influence on the properties of
materials. Introducing impurities has been proved to be an effective approach
to improve the properties of a material and to expand its applications.
However, the interactions between the anionic ISQs and the crystal defects in
electrides are as yet unknown. Here, dense FCC-Li was employed as an archetype
to explore the interplay between anionic ISQs and interstitial impurity atoms
in this electride. This work reveals a strong coupling among the interstitial
impurity atoms, the ISQs, and the matrix Li atoms near to the defects. This
complex interplay and interaction mainly manifest as the unexpected tetrahedral
interstitial occupation of impurity atoms and the enhancement of electron
localization in the interstices. Moreover, the Be impurity occupying the
octahedral interstice shows the highest negative charge state (Be8-) discovered
thus far. These results demonstrate the rich chemistry and physics of this
emerging material, and provide a new basis for enriching their variants for a
wide range of applications.
