Structure searches and superconductor discovery in XB (X = Sc, Ti, V, Cr, and Tc).

With extensive structure searches for XB (X = Sc, Ti, V, Cr, and Tc) under pressures up to 100 GPa, we uncovered that the crystal structures of these compounds with the lowest enthalpy have the same space group (6/) as MgB at ambient pressure. Among them, ScB, TiB and VB are dynamically stable at ambient pressure, but they do not superconduct. CrB becomes dynamically stable at 108 GPa and shows superconductivity with a transition temperature () of 26.0 K. TcB is not dynamically stable until 9 GPa. At 20 GPa, it has a of 23.5 K. Further calculations indicate that CrB and TcB are also thermodynamically stable, suggesting that it is highly likely that they can be synthesized successfully in the laboratory. We found that transition metal atoms (Cr/Tc) dominate soft phonon vibrations and make significant contributions to the electron-phonon coupling (EPC) and superconductivity in CrB/TcB, which is in strong contrast to the case of MgB, where high-frequency B vibrations dominate the EPC and superconductivity. Our work enriches the understanding of superconductivity in transition metal borides.