title: Two-step nucleation mechanism in solid-solid phase transitions.
abstract: The microscopic kinetics of ubiquitous solid-solid phase transitions remain poorly understood. Here, by using single-particle-resolution video microscopy of colloidal films of diameter-tunable microspheres, we show that transitions between square and triangular lattices occur via a two-step diffusive nucleation pathway involving liquid nuclei. The nucleation pathway is favoured over the direct one-step nucleation because the energy of the solid/liquid interface is lower than that between solid phases. We also observed that nucleation precursors are particle-swapping loops rather than newly generated structural defects, and that coherent and incoherent facets of the evolving nuclei exhibit different energies and growth rates that can markedly alter the nucleation kinetics. Our findings suggest that an intermediate liquid should exist in the nucleation processes of solid-solid transitions of most metals and alloys, and provide guidance for better control of the kinetics of the transition and for future refinements of solid-solid transition theory.
authors.affiliation: Department of Physics, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, China.
authors.forename: Yi
authors.initials: Y
authors.lastname: Peng
authors.affiliation: Department of Physics, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, China.
authors.forename: Feng
authors.initials: F
authors.lastname: Wang
authors.affiliation: Department of Physics, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, China.
authors.forename: Ziren
authors.initials: Z
authors.lastname: Wang
authors.affiliation: Complex Assemblies of Soft Matter (COMPASS), Solvay-CNRS-UPenn UMI 3254, Bristol, Pennsylvania 19007, USA.
authors.forename: Ahmed M
authors.initials: AM
authors.lastname: Alsayed
authors.affiliation: Center for Soft Condensed Matter Physics and Interdisciplinary Research, Soochow University, Suzhou 215006, China.
authors.forename: Zexin
authors.initials: Z
authors.lastname: Zhang
authors.affiliation: Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA.
authors.forename: Arjun G
authors.initials: AG
authors.lastname: Yodh
authors.affiliation: 1] Department of Physics, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, China [2] HKUST Shenzhen Research Institute, Shenzhen 518057, China.
authors.forename: Yilong
authors.initials: Y
authors.lastname: Han