Structure searches and superconductor discovery in XB 2 (X = Sc, Ti, V, Cr, and Tc).

With extensive structure searches for XB 2 (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 ( P 6/ mmm ) as MgB 2 at ambient pressure. Among them, ScB 2 , TiB 2 and VB 2 are dynamically stable at ambient pressure, but they do not superconduct. CrB 2 becomes dynamically stable at 108 GPa and shows superconductivity with a transition temperature ( T c ) of 26.0 K. TcB 2 is not dynamically stable until 9 GPa. At 20 GPa, it has a T c of 23.5 K. Further calculations indicate that CrB 2 and TcB 2 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 2 /TcB 2 , which is in strong contrast to the case of MgB 2 , where high-frequency B vibrations dominate the EPC and superconductivity. Our work enriches the understanding of superconductivity in transition metal borides.