Decoupling of Rattling Mode and Superconductivity in Filled-Skutterudite BaxIr4Sb12

The rattling mode, an anharmonic vibrational phonon, is widely recognized as a critical factor in the emergence of superconductivity in caged materials. Here, we present a counterexample in a filled-skutterudite superconductor BaxIr4Sb12 (x = 0.8, 0.9, 1.0), synthesized via a high-pressure route. Transport measurements down to liquid He-3 temperatures reveal a transition temperature (T-c) of 1.2 K and an upper critical field (H-c2) of 1.3 T. Unlike other superconductors with caged structures, the BaxIr4X12 (X = P, As, Sb) family exhibits a monotonic decreasing T-c with the enhancement of the rattling mode, as indicated by fitting the Bloch-Gr & uuml;neisen formula. Theoretical analysis suggests that electron doping from Ba transforms the direct bandgap IrSb3 into a metal, with the Fermi surface dominated by the hybridization of Ir 5d and Sb 5p orbitals. Our findings of decoupled rattling modes and superconductivity distinguish the BaxIr4Sb12 family from other caged superconductors, warranting further exploration into the underlying mechanism.