Phase stability, ordering tendencies, and magnetism in single-phase fcc Au-Fe nanoalloys

TL;DR

This study combines DFT and effective Hamiltonian methods to explore phase stability, ordering, and magnetism in single-phase Au-Fe nanoalloys, revealing persistent phase separation tendencies and incipient ordering influenced by Fe enrichment and surface segregation.

Contribution

It provides a detailed computational analysis of structural energetics and ordering tendencies in Au-Fe nanoalloys, highlighting the instability of certain ordered phases and the role of magnetism and surface effects.

Findings

Phase separation tendency persists despite suppression of fcc-bcc decomposition.

Ordered L1$_0$ and L1$_2$ structures are unstable in DFT calculations.

Incipient ordering occurs with Fe enrichment and surface segregation.

Abstract

Bulk Au-Fe alloys separate into Au-based fcc and Fe-based bcc phases, but L1$_0$ and L1$_2$ orderings were reported in single-phase Au-Fe nanoparticles. Motivated by these observations, we study the structural and ordering energetics in this alloy by combining density functional theory (DFT) calculations with effective Hamiltonian techniques: a cluster expansion with structural filters, and the configuration-dependent lattice deformation model. The phase separation tendency in Au-Fe persists even if the fcc-bcc decomposition is suppressed. The relative stability of disordered bcc and fcc phases observed in nanoparticles is reproduced, but the fully ordered L1$_0$ AuFe, L1$_2$ Au$_3$Fe, and L1$_2$ AuFe$_3$ structures are unstable in DFT. However, a tendency to form concentration waves at the corresponding [001] ordering vector is revealed in nearly-random alloys in a certain range of concentrations. This incipient ordering requires enrichment by Fe relative to the equiatomic composition, which may occur in the core of a nanoparticle due to the segregation of Au to the surface. Effects of magnetism on the chemical ordering are also discussed.