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 Ba x Ir 4 Sb 12 ( x = 0.8, 0.9, 1.0), synthesized via a high-pressure route. Transport measurements down to liquid 3 He 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 Ba x Ir 4 X 12 ( X = P, As, Sb) family exhibits a monotonic decreasing T c with the enhancement of the rattling mode, as indicated by fitting the Bloch–Grüneisen formula. Theoretical analysis suggests that electron doping from Ba transforms the direct bandgap IrSb 3 into a metal, with the Fermi surface dominated by the hybridization of Ir 5 d and Sb 5 p orbitals. Our findings of decoupled rattling modes and superconductivity distinguish the Ba x Ir 4 Sb 12 family from other caged superconductors, warranting further exploration into the underlying mechanism.