Enhanced Superconductivity and Structural Phase Transition in the Solid Solution Rh _{1– <i>x</i>} Pt _<i>x</i> Sb

The solid solution Rh_1-<i>x</i>Pt_<i>x</i>Sb, bridging the structurally distinct MnP-type RhSb and NiAs-type PtSb, was systematically investigated. A temperature-induced structural phase transition between the MnP- and NiAs-type phases occurs at <i>x</i> ≈ 0.1 near room temperature, accompanied by pronounced resistivity hysteresis. Superconductivity emerges for <i>x</i> ≥ 0.2, and the critical temperature (<i>T</i>_c) reaches a maximum value of 4.25 K at <i>x</i> = 0.4─the highest <i>T</i>_c among transition-metal monoantimonides. The electron-phonon coupling constant (<i>λ</i>_ep ≈ 0.6) and normalized specific-heat jump (Δ<i>C</i>_el/<i>γ</i><i>T</i>_c ≈ 1.6) classify Rh_1-<i>x</i>Pt_<i>x</i>Sb as a weak-coupling superconductor. The compositional dependence of both the Debye temperature (Θ_D) and the electronic density of states at the Fermi level (<i>N</i>(<i>E</i>_F)) correlates closely with <i>T</i>_c, suggesting that they are responsible for the <i>T</i>_c enhancement. A comparison with the high-entropy analogue <i>M</i>_1-<i>x</i>Pt_<i>x</i>Sb (<i>M</i> = equimolar Ru, Rh, Pd, and Ir) revealed distinct differences in Θ_D and <i>N</i>(<i>E</i>_F), which can account for the observed difference in <i>T</i>_c between the two systems.