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.

Nonclassical Nucleation in a Solid-Solid Transition of Confined Hard Spheres

“…The equilibrium phase diagram of hard spheres confined between two planar hard walls shows an alternating sequence of solid-solid transitions, …n△ → ðn þ 1Þ□ → ðn þ 1Þ△…, as the plate separation increases [27][28][29][30], where n is the number of crystalline layers. Peng et al [26] found that the transition from the ðn þ 1Þ□ crystal to the ðnÞ△ crystal followed Ostwald's step rule and occurred via a two-step nucleation process involving an intermediate liquid phase.We study the nucleation mechanism of the 5□ → 4△ solid-solid transition in a system of hard spheres of diameter σ confined between two parallel hard plates separated by a distance H=σ ¼ 4, using computer simulations. Our simulations are carried out in the packing fraction range 0.479 < η < 0.500 (i.e., the 2D reduced lateral pressure range 35.8 < P Ã ¼ βPσ 2 < 40), where the 5□ crystal and the liquid phase are metastable with respect to the 4△ crystal (see the Supplemental Material [31] for details).…”

“…The equilibrium phase diagram of hard spheres confined between two planar hard walls shows an alternating sequence of solid-solid transitions, …n△ → ðn þ 1Þ□ → ðn þ 1Þ△…, as the plate separation increases [27][28][29][30], where n is the number of crystalline layers. Peng et al [26] found that the transition from the ðn þ 1Þ□ crystal to the ðnÞ△ crystal followed Ostwald's step rule and occurred via a two-step nucleation process involving an intermediate liquid phase.…”

“…In colloids, martensitic transitions have been observed in small crystalline clusters [20][21][22] or lattices stretched by external fields [18,[23][24][25]. A solid-solid transition involving an activated nucleation process has recently been experimentally observed at the single-particle level for the first time in colloidal thin-film crystals confined between two glass plates [26]. The equilibrium phase diagram of hard spheres confined between two planar hard walls shows an alternating sequence of solid-solid transitions, …n△ → ðn þ 1Þ□ → ðn þ 1Þ△…, as the plate separation increases [27][28][29][30], where n is the number of crystalline layers.…”

Experimental and Theoretical Substantiations of Stepwise Mechanism of Crystal Formation From Liquid Phase

“…Однако некоторые метастабильные кристаллические фазы могут далее последовательно претерпевать серию фазовых превращений «твердое -твердое», например, многие полимеры [2,3]. В последние годы это эмпирическое правило нашло дальнейшее теоретическое и экспериментальное обоснование для последовательности фазовых переходов «метастабильное твердое -метастабильная жидкость -стабильная твердая фаза» [4][5][6][7][8][9]. Образование метастабильной жидкой фазы экспериментально доказано во многих работах при контактном плавление двух металлов [10].…”

“…Образование метастабильной жидкой фазы экспериментально доказано во многих работах при контактном плавление двух металлов [10]. В частности, в работе [8] Ранее кинетическая теория двух-и трех-барьерной нуклеации кристаллов из жидкой фазы была рассмотрена в наших работах [11][12][13]. Было показано, что экспериментальная временная зависимость скорости нуклеации в растворах и в расплавах соответствует пяти предсказанным типам процесса [12,13].…”