Order-disorder transition in Co-Pt nanoparticles: Coexistence, transition states, and finite-size effects

The order to disorder transition in stoichiometric Co-Pt nanoalloys and in a bulk sample has been revisited by Monte Carlo simulations, using the parallel tempering strategy to alleviate slow convergence and circumvent hysteretic behavior. The method is efficient enough to produce coexistence between ordered and disordered phases, as evidenced by the bimodal distributions found for the bulk sample and the 800-atom nanoparticle. This in turn provides some precise evaluation of the transition temperature. By performing committor analyses near relevant regions of configuration space selected by umbrella sampling, we examine how several order parameters allow to characterize the transition state. In addition to a mixing index and a bond-orientational order parameter, a shape-deformation parameter, defined from the deviations from sphericity in the nanoparticles and from the tetragonal distortion in the bulk, is found to separate the ordered and disordered phases most satisfactorily in both cases. While the disordered phase nucleates from local surface defects, it requires significant lattice deformations to proceed completely.