The pursuit of room-temperature superconductivity is a longstanding challenge since the discovery of superconductivity in 1911. Based on the BCS theory, metallic hydrogen could be a room-temperature superconductor, due to its extraordinarily high Debye temperature and very strong electron-phonon interaction. However, metallization and superconductivity of pure hydrogen are yet to be unambiguously demonstrated and may require pressures as high as 5 million atmospheres. In 2004, hydrogen-rich compounds were proposed as alternative candidate materials for exploring high- Tc superconductivity, since the chemical precompression induced by the incorporated elements could significantly lower the metallization pressure of hydrogen to experimentally reachable conditions. The success of this strategy is exemplified by the discovery of high-T_c superconductivity in numerous binary hydrogen-rich compounds, including H₃S (hydrogen atoms form strong covalent bonds with sulfur) and LaH₁₀ (hydrogen atoms form clathrate-type structures). However, these superhydrides remain stable only at extreme pressures, which typically exceed 150 GPa, prompting interest in the design and synthesis of high-T_c superhydrides at lower pressures.
With the additional degrees of freedom, ternary or polynary hydrides are as means both to increase T_c and to enhance stability under pressure. However, the construction of complete phase diagrams of these polynary hydrides is very challenging. Here, we will provide some strategies on designing ternary high-T_c superconducting hydrides. The following systems have been designed: CaYH₁₂,¹ LaBH_x,² and A15-type ternary hydrides.^3-5 Guided by these theoretical results, we successfully synthesized a ternary lanthanum borohydride,⁶ forming robust B-H covalent bonds that lower the pressure required to stabilize the superconducting phase. Electrical transport measurements confirm the presence of superconductivity with a T_c of up to 106 K at 90 GPa, as evidenced by zero resistance and T_c shift under an external magnetic field. X-ray diffraction and transport measurements identify the superconducting compound as LaB₂H₈, a nonclathrate hydride, whose crystal structure remains stable at pressures as low as ~ half megabar.

Superconductors, Hydrides, DAC, First-principles calculations