  A high-throughput screening based on first-principles calculations was
performed to search for new ternary inorganic electrides. From the available
materials database, we identified three new thermodynamically stable materials
(Li$_{12}$Mg$_3$Si$_4$, NaBa$_2$O and Ca$_5$Ga$_2$N$_4$) as potential
electrides made by main group elements, in addition to the well known mayenite
based electride (C12A7:$e^-$). Different from those conventional inorganic
electrides in which the excess electrons play only the role of anions, the
three new materials, resembling the electrides found in simple metals under
high pressure, possess mixed ionic and metallic bonding. The interplay between
two competing mechanisms, together with the different crystal packing motifs,
gives rise to a variety of geometries in anionic electrons, and rich physical
phenomena such as ferromagnetism, superconductivity and metal-insulator
transition. Our finding here bridges the gap between electrides found at
ambient and high pressure conditions.
