Prediction of superconducting ternary hydride MgGeH₆: from divergent high-pressure formation routes

Abstract

Invigorated by the high temperature superconductivity in some binary hydrogen-dominated compounds, we systematically explored high-pressure phase diagrams and superconductivity of a ternary Mg-Ge-H system using ab initio methods. Stoichiometric MgGeH₆ with high hydrogen content exhibiting Pm3[combining macron] symmetry was predicted from a series of high-pressure synthesis paths. We performed an in-depth study on three distinct formation routes to MgGeH₆, i.e., Mg + Ge + 3H₂ → MgGeH₆, MgGe + 3H₂ → MgGeH₆ and MgH₂ + GeH₄ → MgGeH₆ at high pressures. By directly squeezing three elemental solids Mg + Ge + 3H₂, we obtained ternary MgGeH₆ at 200 GPa. By adding a little bit of the MgGe alloy into hydrogen, we found that MgGeH₆ can form and stabilize at about 200 GPa. More intriguingly, upon compressing MgH₂ and GeH₄ to 250 GPa, we also predicted the same MgGeH₆. Electron structure calculations reveal that the cubic MgGeH₆ is a good metal and takes on ionic character. Electron-phonon coupling calculation reveals a large λ = 1.16 for MgGeH₆ at 200 GPa. In particular, we found that ternary MgGeH₆ could be a potential high temperature superconductor with a superconducting transition temperature T_c of ∼67 K at 200 GPa.