Electronic structure and superconductivity in hexagonal Li₃B₂ and Li₂B₂H phases under pressure

The relatively low-pressure layered hexagonal Li₃B₂ (h-Li₃B₂) phase under 20 GPa was identified on the basis of ab initio calculations. Inspired by the high-temperature superconductivity of metallic hydrogen, this study obtained a novel hexagonal Li₂B₂H (h-Li₂B₂H) structure by replacing Li₂ with H. Contrast to h-Li₃B₂, h-Li₂B₂H may be directly synthesized by α-LiB and H₂ under pressure. The two phases exhibited electronic features similar to those in MgB₂ and were expected to be good superconductors. Electron–phonon coupling calculations illustrated that the layered h-Li₃B₂ structure is superconducting at 2 K with λ = 0.39 under 20 GPa. Moreover, h-Li₂B₂H was predicted to possess a large electron–phonon coupling parameter λ of approximately 0.81 and a resulting superconducting critical temperature about 28 K at 20 GPa. The introduced H in the h-Li₂B₂H structure had strong vibrational energy due to its light atomic mass and could enhance the energy of σ and π bonds by bonding with Li. Thus, the introduced H in the h-Li₂B₂H structure is important due to its enhanced electron–phonon coupling and superconducting property.