We report the discovery of a family of ternary platinum phosphides APt 3P (A=Ca, Sr, and La), which crystallize in an antiperovskite-based structure closely related to that of the heavy fermion superconductor CePt 3Si. All three phosphides showed superconductivity at low temperatures and the highest critical temperature T c=8.4K was observed for SrPt 3P. The analysis of specific heat C(T) for SrPt 3P shows clear evidence for very strong coupling s-wave superconductivity with a large ratio between superconducting gap Δ 0 and T c, 2Δ 0/k BT c∼5, and the presence of low-energy phonons. The presence of multiple Fermi surface pockets was inferred from the nonlinear magnetic field dependence of Hall resistivity, which we argue might play a role in realizing the strong coupling of charge carriers with the low-lying phonons.
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