Electride states and superconductivity in unconventional stoichiometric aluminum borides

Abstract

Metal borides (MBs) have recently sparked enormous interest owing to their fascinating structures, superior properties, and widespread applications, including superconductivity. However, metal-bearing borides featuring both interstitial anionic electrons (IAEs) and superconductivity, remain scarce. Here, leveraging comprehensive first-principles structure prediction, we identified several abnormal stoichiometric aluminum-boron phases induced by compression, i.e., stable phases Al3B2 and AlB4 and metastable phases AlB and AlB6. Strikingly, unlike other binary MBs, the unexpected coexistence of zero-dimensional IAEs located at the center of an Al6 octahedron and superconductivity is uncovered in compressed Al3B2 with a B dimer, and the IAEs experience a rare presence-to-disappearance transition during decompression. Electron-phonon coupling (EPC) simulations demonstrate that both Al intercalation compounds AlB4 with bilayer boron and AlB6 with trilayer boron have higher superconducting critical temperatures (Tc=9.9 K for AlB4, 7.2 K for AlB6) than 5.6 K for Al3B2 at 1 atm, which is primarily attributed to the strong coupling between the B 2p electron states at the Fermi level and the high-frequency B-associated phonon modes. Remarkably, the Tc of ambient-pressure AlB4 can be further improved to 28.7 K by an electron doping strategy that induces significant phonon modes softening and further enhances the EPC strength. In this paper, we provide a fresh perspective for MBs and point the way to the discovery of emerging boron-based materials containing metals.

Source-reported materials — not catalogue approval

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