In these equations, ΔGfus denotes the Gibbs free energy of CaH4 fusion (CaH4(s) ↔ CaH4(l)), whereas ΔGhyd represents that of the CaH2 hydrogenation reaction, CaH2(s) + H2(l) ↔ CaH4(s). The symbols (s) and (l) indicate the solid and liquid phases, respectively. γA-B represents the energy of the interface between A and B. Since ΔGhyd becomes negative under high pressure, where the hydrogenation reaction occurs, the surface melting is promoted, as described by Eq. ( ). As a first approximation, here, we focus only on the enthalpies of the homogeneous system of CaH2, CaH4, and H2, neglecting the entropy and interfacial energy As shown in Fig. (b), ΔHfus+ΔHhyd is lower than ΔHfus at pressures above 30 GPa. Consequently, surface melting via hydrogenation could occur at temperatures below the melting point of CaH4 at pressures over 30 GPa. Therefore, liquid CaH4 became solid CaH4 phase after hydrogenation, since the driving force, hydrogenation enthalpy, was no longer applied to the system. Figure (c) shows the enthalpy changes at each stage of CaH2 hydrogenation via surface melting.