Superconductors in the PtPb₃Bi Structure Type

The quest for new superconductors is of both fundamental and technological importance. Recently, an artificial intelligence method correctly predicted PtPb₃Bi to be a superconductor. In this work, we find superconductivity in the newly synthesized MPb_4-x Bi _x (M = Au, Pd, and Rh), of which PtPb₃Bi is a member. When M = Ni, whose radius is considerably smaller, the structure instead collapses into the different Pb-substituted NiBi₃ type. Interestingly, the stoichiometric parameter x shifts across the three compounds to keep the total valence-electron count close to 20 per formula unit. The superconducting transitions occur at 4.9, 4.2, and 3.4 K, for M = Au, Pd, and Rh, respectively. Using electrical resistivity, magnetization, and specific heat measurements, we establish the bulk nature of the superconducting state and determine the critical fields, characteristic length scales, and anisotropy ratios. All three compounds are moderately anisotropic conventional type-II superconductors, with modest upper critical field anisotropies of H _c2 ^∥c /H _c2 ^⊥c ≈ 1.2 to 1.5. These results establish MPb_4-x Bi _x as a family of anisotropic superconductors and a platform for studying how site disorder and Pb-Bi mixing govern superconductivity in heavy-element intermetallics.