Actinium Hydrides AcH₁₀, AcH₁₂, and AcH₁₆ as High-Temperature Conventional Superconductors

The stability of numerous unexpected actinium hydrides was predicted via the evolutionary algorithm USPEX. The electron-phonon interaction was investigated for the hydrogen-richest and most symmetric phases: R3̅ m-AcH₁₀, I4/ mmm-AcH₁₂, and P6̅ m2-AcH₁₆. Predicted structures of actinium hydrides are consistent with all previously studied Ac-H phases and demonstrate phonon-mediated high-temperature superconductivity with T_C in the range of 204-251 K for R3̅ m-AcH₁₀ at 200 GPa and 199-241 K for P6̅ m2-AcH₁₆ at 150 GPa, which was estimated by directly solving the Eliashberg equation. Actinium belongs to the series of d¹ elements (Sc-Y-La-Ac) that form high- T_C superconducting (HTSC) hydrides. Combining this observation with previous predictions of p⁰-HTSC hydrides (MgH₆ and CaH₆), we propose that p⁰ and d¹ metals with low-lying empty orbitals tend to form phonon-mediated HTSC metal polyhydrides.