Unveiling Pressure-Induced Superconductivity in Radium Hydrides: A First-Principles Approach

Abstract

The radium-hydrogen system represents a largely unexplored region of high-pressure chemistry. Using evolutionary crystal structure searches and first-principles calculations, we have systematically explored the Ra-H phase diagram under pressure up to 50 GPa. This computational analysis reveals a series of stable stoichiometries: tetragonal Ra₂H₄ (P4/nmm) and RaH₆ (I4/mmm) cubic RaH (Fm\(\stackrel{-}{3}\)m), and hexagonal RaH₂ (P6/mmm). All phases are confirmed to be thermodynamically, mechanically, and dynamically stable under the corresponding pressures. Hexagonal RaH₂ exhibits outstanding mechanical properties, with a calculated Vickers hardness of ∼20 GPa, indicating its potential as a hard compound. Electronic structure analysis reveals a clear distinction: RaH, and RaH₂ are metallic, whereas Ra₂H₄ and RaH₆ are semiconducting. We further examined the superconducting properties of the metal phases. Cubic RaH emerges as a conventional superconductor, characterized by a substantial electron-phonon coupling constant λ = 0.675 leading to an estimated critical temperature T_c of ∼7.4 K. A detailed analysis of partial electronic density of states, phonon spectra, Eliashberg spectral function and phonon linewidths elucidates the microscopic mechanism. The superconductivity in RaH is predominantly driven by strong interaction between electronic H-s states near the Fermi-level and the high-frequency longitudinal optical phonon modes of the hydrogen sublattice. Contributions from the low-frequency acoustic modes associated with radium atoms are comparatively weak. In contrast, hexagonal RaH₂ exhibits a much weak EPC (λ = 0.36), resulting in a negligibly low T_c of ∼0.13 K.