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 H₂ near 100 GPa, and all lie on the convex hull by 200 GPa. MgH₄ contains two H^- anions and one H₂ molecule per Mg²⁺ cation, whereas the hydrogenic sublattices of MgH₁₂ and MgH₁₆ are composed solely of H₂^δ- 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.