Probing nanoscale structural and order/disorder phase transitions of supported Co-Pt clusters under annealing

Abstract

Structural and order/disorder phase transitions in bimetallic ${\text{Co}}_{50}{\text{Pt}}_{50}$ nanoparticles induced by annealing have been investigated combining in situ small and wide angle x-ray scattering methods. The support of Monte Carlo simulations within a semiempirical tight-binding potential provides a realistic description of the CoPt cluster structure in the size (1.5--3.5 nm) and temperature (300--900 K) ranges, in good correlation with the experiments. If a coalescence process of as-grown noncrystalline icosahedral clusters (2 nm) induced by annealing is detected at low temperature $(<600\text{ }\text{K})$, higher temperatures $(>700\text{ }\text{K})$ are necessary to induce, by internal atom rearrangement, the formation of decahedral structure followed by a transition to an fcc structure. We demonstrate that the expected chemical ordering occurs only from particles of fcc structure. The investigation of the chemical order/disorder transition of 3.5 nm-sized nanoparticles reveals a bistability state around 900 K with a bimodal distribution corresponding to a coexistence of almost fully ordered nanocrystals and fully disordered ones, also supported by theoretical calculations, suggesting a first-order transition in clusters as in bulk alloys.