High-temperature superconductivity in Th-B-H systems via nonclathrate design

Ternary hydrides have recently been predicted to exhibit exceptional superconducting properties under high pressure, positioning them as promising candidates for room-temperature superconductivity. In this work, we perform a systematic investigation of the Th-B-H system at 100 and 200 GPa using state-of-the-art structural prediction method combined with first-principles calculations. Our results identified seven thermodynamically stable compounds, namely ThBH, ${\mathrm{ThBH}}₇$, ${\mathrm{ThB}}₂{\mathrm{H}}₁₀$, ${\mathrm{ThB}}₂{\mathrm{H}}₃$, ${\mathrm{ThB}}₂{\mathrm{H}}₁₃$, ${\mathrm{Th}}₂{\mathrm{BH}}₁₆$, and ${\mathrm{ThB}}₆{\mathrm{H}}₆$, in which B atoms are bonded with H atoms, giving rise to diverse structural motifs, including ${\mathrm{BH}}₄$ tetrahedra, ${\mathrm{B}}₂{\mathrm{H}}₈$ (${\mathrm{H}}₄\mathrm{B}\text{\ensuremath{-}}{\mathrm{BH}}₄$) units, ${\mathrm{BH}}₆$ octahedra, an interpenetrating framework constructed from orthogonal zigzag ${\mathrm{BH}}₃$ chains and corrugated B-H layers. In addition to the existence of conventional atomic H, we uncover exotic hydrogen species, such as isolated ${\mathrm{H}}₅$ planar pentagons and ${\mathrm{H}}₄$ pyramidal units. Further electron-phonon coupling calculations reveal that nonclathrate hydride ${\mathrm{Th}}₂{\mathrm{BH}}₁₆$ exhibits a ${T}_c$ of 72 K at 200 GPa, which increases to 102 K upon decompression to 120 GPa. Moreover, the further results show hole doping could enhance superconductivity, leading to an increased ${T}_c$ of 115 K in the isostructural ${\mathrm{Th}}₂{\mathrm{BH}}₁₅$. These findings provide valuable insights for the design and synthesis of ternary hydrides with high-temperature superconductivity, particularly in rare-earth metal hydrides under high pressure.