In summary, superhydride formation occurs via surface melting, where the excess pressure over equilibrium pressure lowers the activation energy needed to form a liquid superhydride phase as an intermediate. Here, the hydrogenation reaction in superhydride formation is thermodynamically governed by the fusion enthalpy of the superhydride products (ΔHfus), the hydrogenation reaction enthalpy (ΔHhyd), and the interfacial energies (γinterface). Therefore, by carefully controlling these thermodynamic properties with the appropriate temperature and pressure, the reaction pathway can be optimized. This understanding aligns well with the experimental observation that the use of BH3NH3 and a metal precursor is effective for superhydride synthesis . In this process, the metal precursor is not hydrogenated until BH3NH3 releases H2 molecules upon heating. This makes the enthalpy of the starting material (i.e., metal and H2 from BH3NH3) less stable than that of a metal hydride and H2 molecules under high pressure. This further reduces the activation energy and promotes the formation of a liquid superhydride phase as an intermediate state. As a result, surface melting is expected to occur even at lower temperatures than that for metal hydrides. *Ryuhei Sato: rsato@material.t.u-tokyo.ac.jp † Shin-ichi Orimo: shin-ichi.orimo.a6@tohoku.ac.jp the previous report . In this reaction, fluorite-type CaH2 is obtained by substituting Ca 2+ with dopants such as La 3+ and Y 3+ . Therefore, it is expected that this substitution plays a similar role with the surface melting or disordering. The synthesis temperature (1073 K) is close to the melting point of CaH2 at ambient pressure. Comparison of the previous study with the present study suggests that it is important for superhydride synthesis to control experimental conditions based on dopants and melting point.