Composition-Driven Structural Transitions and Thermodynamic Behaviors in CunAgm (m+n = 309) Nanoalloys from the Perspective of Atomic Simulations.

Molecular dynamics simulations with an embedded atom method were employed to systematically investigate the structural evolution and thermodynamic properties of CunAgm (m+n = 309) nanoalloys during cooling. Through comprehensive analysis of free energy, shape factor, pair distribution functions, entropy, and atomic packing visualizations, a detailed structural phase diagram was established for these magic-number nanoalloys. The results reveal that phase transformation pathways are strongly composition-dependent: nanoparticles with minimal or substantial Cu content predominantly form core-shell icosahedral configurations at room temperature, with Ag atoms preferentially segregating to the surface, while intermediate compositions lead to defective packing patterns with high structural disorder. The formation energy convex hull shows a distinct minimum at approximately 82 atom % Cu, indicating maximum thermodynamic stability for intermediate compositions under nanoscale constraints. Entropy analysis demonstrates a general decreasing trend with increasing Cu content, although it exhibits significant nonlinear variations at phase boundaries, directly reflecting differences in atomic arrangement between distinct structural phases. The identification of seven distinct structural transformation pathways, including direct crystallization routes and two-step transitions through intermediate phases, provides a comprehensive understanding of the structural polymorphism in these nanoalloys. Furthermore, detailed analysis of Mackay, anti-Mackay, and chiral icosahedral structures reveals how variations in the outer shell packing directly influence atomic coordination environments and system energetics.