Surface Melting Promoted by Hydrogenation: Guidelines for Superhydride Synthesis

The synthesis of new superhydrides with high superconducting Tc is challenging owing to the high temperatures and pressures required. Herein, we use machine-learning potential molecular dynamics simulations to investigate the initial stages of superhydride formation in calcium hydrides. Upon contact with high-pressure H2, the surface of CaH2 melts, leading to CaH4 formation. This surface melting, facilitated by the negative enthalpy of the hydrogenation reaction, proceeds via a liquid CaH4 intermediate state. Our findings show that surface melting becomes more favorable than the melting of the superhydride bulk under high pressure, reducing the activation energy required for hydrogenation. From these thermodynamics, we propose superhydride synthesis guidelines based on bulk properties: superhydride melting temperature and pressure-dependent hydrogenation enthalpy, readily determined through supplementary calculations during structure prediction workflows.