High critical field superconductivity at ambient pressure in MoB 2 stabilized in the P6/mmm structure via Nb substitution
Recently it was discovered that, under elevated pressures, 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 $\text{R}\bar{3}$m structure to an MgB 2 -like P6/mmm structure. This suggests that remarkably high T c values among diborides are not restricted to 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 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 $\text{R}\bar{3}$m at high temperatures across the Nb 1–x Mo x B 2 series. X-ray diffraction confirms that the synthesized Nb-substituted MoB 2 adopts the MgB 2 crystal structure. Finally, high magnetic field electrical resistivity measurements and specific heat measurements demonstrate that Nb x Mo 1–x B 2 exhibits superconductivity with T c as high as 8 K and critical fields approaching 6 T.