Electride states and superconductivity in unconventional stoichiometric aluminum borides

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 $\mathrm{A}{\mathrm{l}}_{3}{\mathrm{B}}_{2}$ and ${\mathrm{AlB}}_{4}$ and metastable phases AlB and ${\mathrm{AlB}}_{6}$. Strikingly, unlike other binary MBs, the unexpected coexistence of zero-dimensional IAEs located at the center of an $\mathrm{A}{\mathrm{l}}_{6}$ octahedron and superconductivity is uncovered in compressed $\mathrm{A}{\mathrm{l}}_{3}{\mathrm{B}}_{2}$ 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 ${\mathrm{AlB}}_{4}$ with bilayer boron and ${\mathrm{AlB}}_{6}$ with trilayer boron have higher superconducting critical temperatures (${T}_{c}=9.9$ K for ${\mathrm{AlB}}_{4}$, 7.2 K for ${\mathrm{AlB}}_{6}$) than 5.6 K for $\mathrm{A}{\mathrm{l}}_{3}{\mathrm{B}}_{2}$ 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 ${T}_{c}$ of ambient-pressure ${\mathrm{AlB}}_{4}$ 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.