Crystal structures and phase formation thermodynamics of iron-gold nanoclusters

Alloy nanoparticles are being increasingly used in wide variety of applications (catalysis, contrast enhancement in magnetic resonance imaging, etc.). Knowledge of crystal structure and phase formation of the alloy nanoparticles is critical for these applications. Anomalous thermodynamic behavior and unusual phase formation have been reported for nanoparticles with size below 10 nm. In the present work, inert gas condensation (IGC) has been used to produce Au-Fe nanoclusters of varied compositions with a mean size between 5-10 nm. Here, the nanoclusters below 10 nm display complete solubility between Fe and Au in Fex Au1-x (0.3 < x < 0.8) alloys. The as-deposited clusters are single-crystalline but crystallinity is poor with significant disorder. New structural phases have been obtained for as-deposited clusters that are inconsistent with the equilibrium phase diagram. The as-deposited structures were either bcc for x > 0.65 and fcc for x < 0.65, with lattice parameters expanded relative to elemental and rule-of mixtures values, suggesting the presence of self-interstitials. The as-deposited clusters were ferromagnetic at room temperature.;Heat treatment at 600°C for 15 minutes followed by furnace cooling resulted in the size-dependent transformation of the clusters into additional, non-equilibrium structures that depended on cluster composition, while larger clusters followed bulk behavior. At about 65 atom % Fe, clusters transformed to a well-ordered, single fcc phase with a lattice parameter of 0.363 nm, whereas the phase diagram predicted two-phase equilibrium. The stabilization of a single fcc phase was explained by a thermodynamic analysis. This analysis suggests that the single phase stability in the Fe-Au nanoparticles arises from the fact that the introduction of a phase boundary is energetically opposed. Heat treatment of as-deposited particles results in L12 and L1 0ordered structures near 1:3, 3:1 and 1:1 (Fe:Au) stoichiometries respectively. Magnetically, these nanoparticles display strong ferromagnetic behavior for Fe-rich solid solutions and for the 3:1 L12 structure, and weak ferromagnetic behavior, possibly due to a combination of antiferromagnetic core and uncompensated spins at the surface for other compositions.