Metallization of Magnesium Polyhydrides Under Pressure

TL;DR

This study predicts new magnesium hydride phases under high pressure, revealing their structures, stability, and potential for high-temperature superconductivity, expanding understanding of hydrogen-rich materials.

Contribution

The paper introduces previously unknown stable magnesium polyhydride phases with unique structures and superconducting properties under high pressure.

Findings

MgH4, MgH12, and MgH16 are stable near 100 GPa and on the convex hull by 200 GPa.

MgH12 exhibits a high $T_c$ nearly three times that of MgH2 at similar pressures.

High hydrogen content phases have large density of states at the Fermi level.

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.