Pressure induced high-Tc superconductivity in ternary SrBH8 with distinctive B–H motif and complex hydrogen bonding patterns

Inspired by the recent discovery of near or even above room-temperature superconductors in the pressurized H${}_3$S, CaH${}_6$, YH${}_9$, LaH${}_{10}$, and CeH${}_{18}$, H-rich hydrides under high pressure are believed to be the potential candidates for high-temperature superconductors. Unfortunately, the tremendous pressures needed to stabilize these structures exceptionally restrict their practical applications. An effective strategy to reduce the external pressure is to incorporate the light boron atom into established binary hydrides to construct binary B–H textures with the B–H covalent bonds. Guided by this hypothesis, crystal structures and superconductivity of ternary Sr-B-H system under pressure were theoretically delved into. Notably, we successfully identify a stable SrBH₈ under 137–300 GPa and the triclinic P-1-SrBH₈ phase can be retained to at least 137 GPa during decompression. Subsequently, trigonal R-3m-SrBH₈ becomes more stable than P-1-SrBH₈ at 212 GPa. Intriguingly, P-1-SrBH₈ consists of the BH₄ framework and H₂ molecular dimer while R-3m-SrBH₈ encompasses the BH₆ motif and H₄ unit. Namely, despite the H atom bonding mode bonded to B atom in BH₄ and BH₆ units in both phases is same with robust B–H covalent bond, the remanent H atom bonding modes are distant, which denotes the H₂ unit with the H2–H2 covalent bond and weak H1–H2 covalent bond in the H4 texture emerge in P-1-SrBH₈ and R$-3$m-SrBH₈, respectively. Both crystals are predicted to be promising superconductors with the estimated T ${}_c$s of 103 K (P-1-SrBH₈, 137 GPa) and 141 K (R$-3$m-SrBH₈, 300 GPa), which are separately attributed to the vibration of the B–H bond in BH₄ framework, and a hybrid vibration of the B–H bond in BH₆ building and the H1–H2 bond in H₄ unit. This work provides an effective strategy for the design of the low-pressure stabilized high-temperature hydride superconductors, which is destined to widen the explorations on the particular B–H skeleton and abundant H bonding mode in the H-rich ternary borohydrides.