Low-pressure superconductivity of Th-B-H ternary compounds: An insightful study of stable and metastable phases

The low-enthalpy structures of Th-B-H ternary compounds below 50 GPa are investigated by using an efficient framework that combines high-throughput screening, genetic algorithms, and first-principles calculations. Four thermodynamically stable hydrides (Th₂BH₁₁, Th₂BH₁₀, ThB₂H₁₀, and ThBH) and seven thermodynamically metastable hydrides (ThBH₂, ThBH₃, ThBH₄, ThBH₅, ThBH₇, ThB₂H₄, and ThB₃H₃) are revealed. There are no stable thorium boron hydrides at ambient pressure. R3m-ThB₂H₁₀ and R3m-Th₂BH₁₁ are indirect bandgap semiconductors, while R3m-Th₂BH₁₀ and P6/mmm-ThBH show metallic nature. The relatively low λ values of R3m-Th₂BH₁₀ and P6/mmm-ThBH result in their T_c around 5.8 K and 0.1 K at 20 GPa, respectively. Two metastable phases, $F\overline{4}3m$-ThBH₅ and P6₃/mmc-ThBH₇, exhibit excellent superconducting properties at low pressures. The values of T_c of $F\overline{4}3m$-ThBH₅ and P6₃/mmc-ThBH₇ reach 46.9 K at 20 GPa and 40.0 K at 50 GPa, respectively. Our findings offer valuable insights for discovering and designing superconducting rare earth hydrides under low and even ambient pressures.