Structure searches and superconductor discovery in XB₂ (X = Sc, Ti, V, Cr, and Tc)

With extensive structure searches for XB₂ (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 (P6/mmm) as MgB₂ at ambient pressure. Among them, ScB₂, TiB₂ and VB₂ are dynamically stable at ambient pressure, but they do not superconduct. CrB₂ becomes dynamically stable at 108 GPa and shows superconductivity with a transition temperature (T_c) of 26.0 K. TcB₂ is not dynamically stable until 9 GPa. At 20 GPa, it has a T_c of 23.5 K. Further calculations indicate that CrB₂ and TcB₂ 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 CrB₂/TcB₂, which is in strong contrast to the case of MgB₂, where high-frequency B vibrations dominate the EPC and superconductivity. Our work enriches the understanding of superconductivity in transition metal borides.