Metallization of Magnesium Polyhydrides Under Pressure

Abstract

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${}^{-}$ 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${}_2^{\delta -}$ 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.

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