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.