Increased Superconducting Transition Temperature and Upper Critical Field of a High-Entropy Antimonide Superconductor (RuRhPdIr)_1–<i>x</i>Pt_<i>x</i>Sb

High-entropy compounds have garnered significant interest in recent years owing to their exceptional properties and functionalities derived from the cocktail effect, which indicates that the properties of high-entropy compounds will be significantly enhanced compared to the average properties obtained from the individual properties of the constituent elements. Herein, we report an increased superconducting transition temperature (Tc) and upper critical field (Hc2) as a new cocktail effect in the high-entropy antimonide superconductor (RuRhPdIr)1–xPtxSb, where x denotes the Pt content. Transport measurements revealed a composition-dependent systematic change from unusual transport properties due to the extremely strong scattering of electrons and phonons for x = 0.2 to a normal metallic state for x = 1 (PtSb). Tc also varied with the Pt content, which could be attributed to the change in the electron–phonon coupling, reaching a maximum of 3.1 K at x = 0.4. This was the highest value among the transition metal mono antimonides. The Hc2 value for x = 0.2 was 7 times higher than that for x = 1, which is likely attributed to the shortening of the coherence length due to chemical disorder. This study demonstrates that extremely strong chemical disorders in high-entropy compounds may be effective in improving their superconducting properties.