Size-Effect Enriched Phase Diagram in p-Type Skutterudite Superconductor Ir_3.8Sb₁₂

Pressurized Ir₄X₁₂ (X = P and As) stands out as the sole p-type superconductors among dozens of filled-skutterudites that are primarily characterized by n-type charge carriers. The emergence of superconductivity is proposed to be intimately related to the inharmonic rattling phonons originating from the filled atoms. Here, we explore the impact of the size effect of the rattling atoms by substituting X with Sb, whose radius is 30 and 17% larger than those of P and As, respectively. In situ synchrotron X-ray diffractions reveal a similar structural evolution of self-inserted Sb atoms inside the Sb₁₁ cage but at a significantly lower critical pressure of 33.6 GPa. Interestingly, single crystalline Ir_3.8Sb₁₂ exhibits a more diverse electronic phase diagram compared to its P- and As-based counterparts, encompassing an anomalous resistance upturn, a complete superconducting dome, and a filled-semiconducting region. We propose that the intrinsic deficiency of Ir and the filling ratio of central atoms, both stemming from the size effect of Sb, are responsible for the observed rich phenomena.