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. MgH${}_{4}$, MgH${}_{12}$, and MgH${}_{16}$ are found to be thermodynamically stable with respect to decomposition into MgH${}_{2}$ and H${}_{2}$ near 100 GPa, and all lie on the convex hull by 200 GPa. MgH${}_{4}$ contains two H${}^{\ensuremath{-}}$ anions and one H${}_{2}$ molecule per Mg${}^{2+}$ cation, whereas the hydrogenic sublattices of MgH${}_{12}$ and MgH${}_{16}$ are composed solely of H${}_{2}^{\ensuremath{\delta}\ensuremath{-}}$ molecules. The high-hydrogen content stoichiometries have a large density of states at the Fermi level, and the ${T}_{c}$ of MgH${}_{12}$ at 140 GPa is calculated to be nearly three times greater than that of the classic hydride, MgH${}_{2}$, at 180 GPa.