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.
Categories: cond-mat.mtrl-sci