Surface melting-driven hydrogen absorption for high-pressure polyhydride synthesis

The synthesis of new polyhydrides with high superconducting T_c is challenging owing to the high pressures and temperatures required. In this study, we used machine-learning potential molecular dynamics simulations to investigate the initial stage of polyhydride formation in calcium hydrides. Upon contact with high-pressure H₂, the surface of CaH₂ melts, leading to CaH₄ formation. This surface melting proceeds via CaH₄ liquid phase as an intermediate state. High pressure reduces not only the hydrogenation (CaH₂(s) + H₂(l) ↔ CaH₄(s)) enthalpy but also the enthalpy for liquid polyhydride formation (CaH₂(s) + H₂(l) ↔ CaH₄(l)). Consequently, this surface melting process becomes more favorable than the fusion of the polyhydride bulk. Thus, high pressure not only shifts the equilibrium toward the polyhydride product but also lowers the activation energy, thereby promoting the hydrogenation reaction. From these thermodynamic insights, we propose structure-search criteria for polyhydride synthesis that are both computationally effective and experimentally relevant. These criteria are based on bulk properties, such as polyhydride (product) melting temperature and pressure-dependent hydrogenation enthalpy, readily determined through supplementary calculations during structure prediction workflows.