Electrochemical hydrogen insertion as a candidatenon-equilibrium route from Mg2IrH5 to thepredicted ambient-pressure superconductorMg2IrH6: a DFT reproduction and a proposal

Mg2IrH6 is a predicted ambient-pressure superconductor (Tc ≈ 80–160 K) that has notyet been synthesized; equilibrium synthesis instead yields the charge-balancedinsulator Mg2IrH5 [3]. Using plane-wave DFT on a desktop computer, I reproduce thekey published results: the insertion reaction Mg2IrH5 + ½H2 → Mg2IrH6 is uphill (bareDFT ΔE = +0.66 eV per hydrogen, 74 meV/atom, consistent with the reported hulldistance of ~60 meV/atom), Mg2IrH5 is insulating while Mg2IrH6 is metallic, insertionoccurs at essentially constant volume (ΔV < 0.1%), and 34% of the Fermi-leveldensity of states of Mg2IrH6 derives from hydrogen. On this basis I proposeelectrochemical hydrogen insertion as a third non-equilibrium route to Mg2IrH6,alongside the deposition and implantation routes suggested in Ref. [3]. Afterestimated zero-point and entropy corrections the required potential is roughly −1.0 Vversus the hydrogen electrode at room temperature (−0.85 V under cold operation)— demanding, and its feasibility hinges on the unknown surface chemistry of thehydride — its hydrogen-evolution activity and its acid–base reactivity toward protondonors; complete suppression of hydrogen evolution is however not required. ARaman fingerprint (Ir–H stretch, 2108 → ~1850 cm−1) provides the success criterion.