Element-specific quantitative determination of the local atomic order in CoPt alloy nanoparticles: Experiment and theory

Abstract

An annealing-driven transition from a chemically disordered $A1$-like structure to a chemically ordered $L{1}_{0}$-like phase has been revealed for size-selected CoPt clusters with diameters from 2 to 4 nm, from experiment to theory confrontation. For chemically ordered particles, atypical lattice-parameter ratios $c/a$ are inferred. This ratio is found to be remarkably different depending on the approach used ($c/a>1$ from x-ray absorption at the Co $K$ edge and $c/a\ensuremath{\simeq}0.94$ from the Pt dominated x-ray diffraction). A first-principles theoretical study accurately explains this astonishing feature and provides a detailed understanding of the element-specific local relaxations, which are crucial in these nanoalloys.