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

Synthesis of novel inorganic materials involving molecular absorption is challenging owing to the control of interfacial reactions. Here, we reveal the kinetic role of applied pressure in the interfacial reaction. Specifically, 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. This surface melting proceeds via CaH4 liquid phase as an intermediate state. Therefore, the high pressure reduces not only the hydrogenation (CaH2 + H2 ↔ CaH4) enthalpy but also the activation energy of the reaction via liquid product phase. This advantage of high-pressure synthesis is applicable to any interfacial reactions involving molecular absorption, opening new avenue to inorganic material synthesis. From these thermodynamics, we also propose superhydride synthesis guidelines based on bulk properties: superhydride (product) melting temperature and pressure-dependent hydrogenation enthalpy, readily determined through supplementary calculations during structure prediction workflows.