Superconductivity at 9 K in Mo5PB2 with evidence for multiple gaps

Superconductivity is observed with critical temperatures near 9 K in the tetragonal compound Mo5PB2. This material adopts the Cr5B3 structure type common to superconducting 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 ≈17 kOe. Together, the measurements and band-structure calculations indicate intermediate coupling (λ=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.