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

## TL;DR

This study explores the structure and composition of Ni–Cu–Fe-Co nanoparticles using experiments and simulations, revealing Cu-rich shells and Ni/Fe cores.

## Contribution

The paper introduces combined experimental and computational insights into the hierarchical structure of quaternary Ni–Cu–Fe-Co nanoparticles.

## Key findings

- Cu enrichment of ≈25–30% occurs on nanoparticle surfaces, while Ni and Fe concentrate in cores.
- Melting temperatures increase with nanoparticle size, while crystallization temperatures decrease with faster cooling rates.
- Surface and potential energies align with observed Cu segregation and structural stability.

## Abstract

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.