Predicted stable electrides in Mg-Al systems under high pressure.

Magnesium and aluminum, as adjacent light metal elements, have difficulty forming stable stoichiometric compounds under ambient conditions. In this work, using evolutionary structural prediction approaches, we have systematically explored the entire compositional space to identify possible stoichiometries of Mg-Al compounds at pressures of up to 100 GPa. Here, three compounds, MgAl, MgAl and MgAl, are predicted to be thermodynamically stable at certain pressure conditions. In particular, we discovered that these newly found compounds are essentially electrides with excess electrons confined in the interstitial voids, which is attributed to the 3p orbitals of the two nearest-neighbor Al atoms overlapping under high pressure. This work will provide an avenue for further experimental study of the new structures of Mg-Al systems, and will also have far-reaching implications on the understanding of materials chemistry under high pressure.