Ordering Frustration In Large-Scale Co-Pt Nanoalloys

Large-scale nanoalloys of Co-Pt from 3 to 7 nm are modeled using realistic atomistic potentials and Monte Carlo simulations, sweeping the entire concentration range. They display a clear tendency to reproduce bulk-ordered phases in their core- and surface-ordered superstructures on their facets. Most of the ordered facets are simply the termination of the core-ordered phase but one of them, namely, (root 3 x -root 3) R30 degrees(111), is not compatible with the core structure, inducing disorder underneath (111) facets. At equiconcentration, the LI0 phase, within the truncated octahedral shape, displays (100) facets leading to both pure and mixed variants with no surface rearrangements. However, we predict mixed composition on all (100) facets, as for the infinite (100) surfaces, indicating a chemical ordering frustration in nanoalloys. For 5-fold symmetric structures like decahedra and icosahedra, the core ordering remains inside the multiply twinned tetrahedra forming the decahedral or icosahedral structure.