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.