Metallization of magnesium polyhydrides under pressure

Evolutionary structure searches are used to predict stable phases with unique stoichiometries in the hydrogen-rich region of the magnesium/hydrogen phase diagram under pressure. MgH4, MgH12, and MgH16 are found to be thermodynamically stable with respect to decomposition into MgH2 and H-2 near 100 GPa, and all lie on the convex hull by 200 GPa. MgH4 contains two H- anions and one H-2 molecule per Mg2+ cation, whereas the hydrogenic sublattices of MgH12 and MgH16 are composed solely of Hd(2)(delta-) molecules. The high-hydrogen content stoichiometries have a large density of states at the Fermi level, and the T-c of MgH12 at 140 GPa is calculated to be nearly three times greater than that of the classic hydride, MgH2, at 180 GPa. DOI: 10.1103/PhysRevB.87.054107