Composition-dependent ordering transformations in Pt-Fe nanoalloys

Despite the well-known tendency for many alloys to undergo ordering transformations, the microscopic mechanism of ordering and its dependence on alloy composition remains largely unknown. Using the example of Pt85Fe15 and Pt65Fe35 alloy nanoparticles (NPs), herein we demonstrate the composition-dependent ordering processes on the single-particle level, where the nanoscale size effect allows for close interplay between surface and bulk in controlling the phase evolution. Using in situ electron microscopy observations, we show that the ordering transformation in Pt85Fe15 NPs during vacuum annealing occurs via the surface nucleation and growth of L1(2)-ordered Pt3Fe domains that propagate into the bulk, followed by the self-sacrifice transformation of the surface region of the L1(2) Pt3Fe into a Pt skin. By contrast, the ordering in Pt65Fe35 NPs proceeds via an interface mechanism by which the rapid formation of an L1(0) PtFe skin occurs on the NPs and the transformation boundary moves inward along with outward Pt diffusion. Although both the "nucleation and growth" and the "interface" mechanisms result in a core-shell configuration with a thin Pt-rich skin, Pt85Fe15 NPs have an L1(2) Pt3Fe core, whereas Pt65Fe35 NPs are composed of an L1(0) PtFe core. Using atomistic modeling, we identify the composition-dependent vacancy-assisted counterdiffusion of Pt and Fe atoms between the surface and core regions in controlling the ordering transformation pathway. This vacancy-assisted diffusion is further demonstrated by oxygen annealing, for which the selective oxidation of Fe results in a large number of Fe vacancies and thereby greatly accelerates the transformation kinetics.