Diboride compounds doped with transition metals: A route to superconductivity through structure stabilization as well as defects

Recent investigations into ${\mathrm{MoB}}₂$ have unveiled a direct connection between a pressure-induced structural transition to a $P6/mmm$ space group structure and the emergence of superconductivity, producing critical temperatures up to 32 K at 100 GPa. This pressure-induced superconducting state underscores the potential of doped ${\mathrm{MoB}}₂$ as a possible candidate for metastable superconductivity at ambient pressure. In this work, we demonstrate that doping by Zr, Hf, or Ta stabilizes the $P6/mmm$ structure at ambient pressure and results in the realization of a superconducting state with critical temperatures ranging from 2.4 up to 8.5 K depending on the specific doping. We estimate the electron-phonon coupling $\ensuremath{\lambda}$ and the density of states based on resistivity and specific heat data, finding that $\ensuremath{\lambda}$ ranges from 0.4 to 0.6 for these compounds. Finally, to investigate the role of possible metastable defect structures on the critical temperature, we analyze ${\mathrm{MoB}}₂, {\mathrm{MoB}}_2.5$, and Nb/Zr-doped ${\mathrm{MoB}}₂$ using rapid cooling techniques. Notably, splat quenching produces samples with higher critical temperatures and even retains superconductivity in ${\mathrm{MoB}}₂$ at ambient pressure, achieving a critical temperature of 4.5 K.