Hydrogen Absorption Promoted by Surface Melting: Guidelines for High-Pressure Superhydride Synthesis

The synthesis of new superhydrides with high superconducting Tc is challenging due to the high temperatures and pressures required. Here, we used machine-learning potential molecular dynamics simulations to investigate the initial stage of superhydride formation in calcium hydrides. Upon contact with high-pressure H2, the surface of CaH2 melts, leading to CaH4 formation. High pressure reduces the formation enthalpy for liquid superhydride as an intermediate state. Consequently, excess pressure above equilibrium shifts the balance towards superhydride formation and lowers the activation energy, promoting the hydrogenation reaction. Based on these thermodynamic insights, we propose superhydride synthesis guidelines focused on bulk properties: superhydride (product) melting temperature and pressure-dependent hydrogenation enthalpy, readily determined through supplementary calculations during structure prediction workflows.