Superconductivity in the Superhard Boride WB${}_{4.2}$

We show that the superhard boride WB${}_{4.2}$ is a superconductor with a T${}_c$ of 2.05(5) K. Temperature-dependent magnetic susceptibility, electrical resistivity, and specific heat measurements were used to characterize the superconducting transition. The Sommerfeld constant {\gamma} for WB${}_{4.2}$ is 2.07(3) mJ mol${}^{-1}$ K${}^{-2}$ and the {\Delta}C/{\gamma}T${}_c$ = 1.56, which is somewhat higher than what is expected for weakly coupled BCS type superconductors. The H${}_{c2}$ vs T plot is linear over a wide temperature range but does show signs of flattening by the lowest temperatures studied and therefore the zero-temperature upper critical field ({\mu}${}_0$H${}_{c2}$(0)) for WB${}_{4.2}$ lies somewhere between the linear extrapolation of {\mu}${}_0$H${}_{c2}$(T) to 0 K and expectations based on the WHH model.