The AuCu Phase Diagram at the Nano Scale: A Molecular Dynamics Approach

Abstract

While phase diagrams for metal nanoalloys are important to know the conditions at which a nanoparticle keeps a particular shape and kind of surface, experiments to build phase diagrams at the nanoscale are difficult to implement; a posible alternative is the use of atomistic simulations. In this work, a set of molecular dynamics simulations is implemented to build the phase diagram of icosahedral, cuboctahedral, and decahedral AuCu nanoparticles of 2 to 4 nm in size, with several relative concentrations of the metals, using the quantum-corrected version of the Sutton and Chen interaction model. In the obtained phase diagrams, the congruent melting point shifts towards high concentrations of copper in accordance with previous theoretical results, and the local density mapping and bond-order analysis as function of temperature show that for the largest particles, partial premelting of the surface coexists with a pseudo-crystalline region that shrinks as the temperature is incremented, followed by the nucleation of a melted region in the inner volume of the particle that grows until the particle melts as a whole. After melting, it is found that gold has a tendency to migrate to the surface, but the alloy remains in the whole volume of the nanoparticle.