Compositional Effects on Chemical Ordering, Local Atomic Pressure and Thermal Stability in Truncated Octahedral Pd-Ir-Rh Trimetallic Nanoalloys

This study presents a comprehensive atomistic investigation of the structural, mechanical, and thermal properties of Pd₆₀Ir_nRh_19-n trimetallic nanoclusters adopting a truncated octahedral geometry. The compositional evolution of chemical ordering, local pressure distributions, and melting behavior was systematically analyzed using Gupta potential-based basin-hopping global optimization. The accuracy of the Gupta potential predictions was further validated for all configurations using density functional theory (DFT) calculations. The surface layer consisted solely of Pd atoms and was held constant throughout the study. Meanwhile, Ir and Rh atoms were distributed within the 19-atom core region, allowing a detailed evaluation of how variations in core composition affect the energetic and thermal stability of the clusters. The Pd₆₀Ir₆Rh₁₃ configuration exhibits the minimum value of mixing energy, corresponding to the most symmetric and energetically stable atomic arrangement. Local pressure analyses showed that Ir incorporation enhances internal compressive stress and induces tensile relaxation on the Pd surface, achieving an optimal strain balance at n = 6. Melting analyses based on caloric curves and Lindemann indices revealed a non-monotonic dependence of melting temperature on Ir content, with Ir-rich clusters displaying the highest thermal resistance and Rh-rich systems showing reduced stability. These findings clarify how Ir/Rh distribution governs the energetic, mechanical, and thermal response of Pd-Ir-Rh nanoalloys, offering a coherent atomistic framework for understanding their composition-dependent stability.