Mo$_5$PB$_2$: a new superconductor in the Cr$_5$B$_3$ structure type with $T_c$ = 9.2 K

Superconductivity has been reported recently in several ternary silicide-borides adopting the tetragonal Cr$_5$B$_3$ structure type, including Nb$_5$Si$_{3-x}$B$_x$, Mo$_5$SiB$_2$, and W$_5$SiB$_2$, with critical temperatures ranging from 5.8-7.8 K. Here we report superconductivity with $T_c$ exceeding 9 K in the phosphorus-containing analogue Mo$_5$PB$_2$. We have synthesized polycrystalline samples of the compound, made measurements of electrical resistivity, magnetic susceptibility, and heat capacity, and performed first principles electronic structure calculations. The highest $T_c$ values occur in slightly phosphorus rich samples, with composition near Mo$_5$P$_{1.1}$B$_{1.9}$. Together with the measured properties, the calculations suggest the superconductivity in these materials may be multi-band.