Decoupling of Rattling Mode and Superconductivity in Filled-Skutterudite Ba_xIr₄Sb₁₂

Abstract 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_xIr₄Sb₁₂ (x= 0.8, 0.9, 1.0), synthesized via a high-pressure route. Transport measurements down to liquid ³He 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_xIr₄X₁₂ (X= P, As, Sb) family exhibits a monotonic decreasingT_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 IrSb₃ into a metal, with the Fermi surface dominated by the hybridization of Ir 5dand Sb 5porbitals. Our findings of decoupled rattling modes and superconductivity distinguish the Ba_xIr₄Sb₁₂ family from other caged superconductors, warranting further exploration into the underlying mechanism.