First-principles study on pressure-induced superconductivity and structural design on transition metal diborides.

Recent experiments on α-MoB2 with MgB2-type structure achieved superconductivity at ∼32 K under 90 GPa, the highest among transition-metal diborides, rekindling interest in their superconducting properties. Our study systematically investigates the band structures of AlB2-type transition metal diborides. We found that the superior superconductivity of MoB2, WB2, and TcB2 correlates with their von Hove singularities near the Fermi level (EF), potentially linked to electron-phonon coupling. These three diborides exhibit similar critical temperature (Tc) trends under pressure: rising initially, peaking around 60 GPa, and then declining. While unstable at ambient pressure, their thermodynamic and dynamical stability limits vary significantly, possibly explaining experimental discrepancies. To stabilize MoB2 at ambient pressure, we designed MoXB4 compounds (X = other transition metals) by substituting every other Mo layer in MoB2 with an X layer. This modification aims to stabilize the structure and enhance superconductivity by reducing d-electron concentration at EF. This principle extends to other potential superconducting diborides, such as WB2 and TcB2. Using Nb as an example, we found that Nb atoms in AlB2-type MoNbB4 may exhibit random occupancy, potentially explaining disparities between theoretical predictions and experimental results. Our study offers valuable insights into superconductivity in transition metal diborides, paving the way for future research and applications.