Chemical and Structural Segregation in Quaternary Ni-Cu-Fe-Co Nanoparticles: Atomistic Simulation and Experiment

A comprehensive study of quaternary Ni-Cu-Fe-Co nanoparticles with sizes ranging from 2,000 to 10,000 atoms (≈10-30 nm) was carried out by combining solution combustion synthesis, X-ray diffraction (XRD), transmission electron microscopy (TEM-HAADF-EDS), and atomistic modeling (molecular dynamics and Monte Carlo simulations). Experimental XRD patterns confirmed the predominance of the face-centered cubic (fcc) structure with broadened reflections, indicative of nanocrystalline domains and partial coexistence of hexagonal close-packed (hcp) phases. TEM-EDS analysis showed well-defined crystallites and pronounced surface segregation of Cu (≈25-30%) enrichment relative to bulk composition and partial Co enrichment, in contrast to Ni and Fe, which concentrated in the particle cores. Molecular dynamics simulations showed that the melting temperature (T _m) increases with particle size, from 1371-1379 (2000 atoms) to 1479-1488 K (10,000 atoms), corresponding to an 8.5% rise. Conversely, crystallization temperatures (Tc) decrease with faster cooling, e.g., from 1159 at 0.25 to 1086 at 0.75 K/ps, reflecting kinetic effects on solidification. The potential energy stabilized from -3.98 (2000 atoms) to -4.06 eV/atom (10,000 atoms), while surface energy decreased from 2320-2361 to 2231-2283 mJ/m², in agreement with experimental evidence of Cu segregation. These combined experimental and computational insights reveal that Ni-Cu-Fe-Co nanoparticles inherently form hierarchical, labyrinth-like structures with Cu-rich shells and Ni/Fe-dominated cores.