Electron, phonon, and superconducting properties of Mg2CuH6 under pressure

Recent theoretical prediction of Mg₂IrH₆ is a significant advance in achieving high-temperature superconductivity under atmospheric pressure, Mg₂IrH₆ is the hydride superconductor with the highest superconducting transition temperature (T_c ∼ 160 K) under ambient pressure so far. In general, T_c is usually related to the degree of hydrogen enrichment. As a representative of the hydrogen-poor structures, MgHCu₃ was also predicted to have a T_c of 43 K under atmospheric pressure. In this work, we try to replace the Ir atom in Mg₂IrH₆ with the Cu atom to improve the superconductivity of the system. The phonon dispersion curves and the ab initio molecular dynamics (AIMD) simulation indicate that the novel ternary Mg₂CuH₆ hydride is dynamically and thermodynamically stable. But, regrettably, the T_c and electron-phonon coupling (EPC) parameters λ of Mg₂CuH₆ are calculated to be 12.5 K and 0.55 at 25 GPa, which is far inferior to the superconductivity of Mg₂IrH₆. Compared with $Fm\overline{3}m$ Mg₂CuH₆, $Fm\overline{3}m$ Mg₂IrH₆ produces obvious phonon softening in the G point near 30 meV, which significantly enhances the EPC and increases the T_c of the system. Moreover, Cu substitution for Ir results in a sharp decrease in the contribution of the electronic states for Mg and H to the total density of states at the Fermi level. The huge difference in predicted T_c between Mg₂CuH₆ and Mg₂IrH₆ and their causes may provide insights into the design of high-temperature atmospheric superconductors and help to understand the superconducting mechanisms of related systems.