High critical field superconductivity at ambient pressure in MoB2 stabilized in the P6/mmm structure via Nb substitution

Recently it was discovered that, under elevated pressures, ${\mathrm{MoB}}₂$ exhibits superconductivity at a critical temperature ${T}_c$ as high as 32 K. The superconductivity appears to develop following a pressure-induced structural transition from the ambient pressure $\mathrm{R}\overline{3}\mathrm{m}$ structure to an ${\mathrm{MgB}}₂$-like P6/mmm structure. This suggests that remarkably high ${T}_c$ values among diborides are not restricted to ${\mathrm{MgB}}₂$ as previously appeared to be the case, and that similarly high ${T}_c$ values may occur in other diborides if they can be coerced into the ${\mathrm{MgB}}₂$ structure. In this paper, we show that density functional theory calculations indicate that phonon free energy stabilizes the P6/mmm structure over the $\mathrm{R}\overline{3}\mathrm{m}$ at high temperatures across the ${\mathrm{Nb}}_{1\ensuremath{-}x}{\mathrm{Mo}}_x{\mathrm{B}}₂$ series. X-ray diffraction confirms that the synthesized Nb-substituted ${\mathrm{MoB}}₂$ adopts the ${\mathrm{MgB}}₂$ crystal structure. High magnetic field electrical resistivity measurements and specific heat measurements demonstrate that ${\mathrm{Nb}}_x{\mathrm{Mo}}_{1\ensuremath{-}x}{\mathrm{B}}₂$ exhibits superconductivity with ${T}_c$ as high as 8 K and critical fields approaching 6 T.