  Superconductivity is observed with critical temperatures near 9K in the
tetragonal compound Mo5PB2. This material adopts the Cr5B3 structure type
common to supercondcuting Nb5Si3-xBx, Mo5SiB2, and W5SiB2, which have critical
temperatures of 5.8-7.8 K. 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 Tc value (9.2 K) occurs in slightly phosphorus rich
samples, with composition near Mo5P1.1B1.9, and the upper critical field Hc2 at
T = 0 is estimated to be about 17 kOe. Together, the measurements and band
structure calculations indicate intermediate coupling (lambda = 1.0), phonon
mediated superconductivity. The temperature dependence of the heat capacity and
upper critical field Hc2 below Tc suggest multiple superconducting gaps may be
present.
