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

Abstract

A solid-solid phase transition of colloidal hard spheres confined between two planar hard walls is studied using a combination of molecular dynamics and Monte Carlo simulation. The transition from a solid consisting of five crystalline layers with square symmetry (5□) to a solid consisting of four layers with triangular symmetry (4△) is shown to occur through a nonclassical nucleation mechanism that involves the initial formation of a precritical liquid cluster, within which the cluster of the stable 4△ phase grows. Free-energy calculations show that the transition occurs in one step, crossing a single free-energy barrier, and that the critical nucleus consists of a small 4△ solid cluster wetted by a metastable liquid. In addition, the liquid cluster and the solid cluster are shown to grow at the planar hard walls. We also find that the critical nucleus size increases with supersaturation, which is at odds with classical nucleation theory. The △-solid-like cluster is shown to contain both face-centered-cubic and hexagonal-close-packed ordered particles. The kinetics of phase transitions plays an important role in condensed-matter physics and materials science. In order to gain a better fundamental understanding of how to control self-assembly processes in the fabrication of novel structures, many experimental and simulation studies have been devoted to colloidal systems. Experiments [1-4] and computer simulations [5][6][7] on bulk hard-sphere colloids suggested that the metastable fluid crystallizes and superheated crystals melt via a single-step nucleation process that is well described by classical nucleation theory (CNT) [8]. However, Ostwald's step rule suggests that the kinetic pathway to the most stable state can initially proceed through the nucleation of intermediate, metastable phases [9]. The effect of a nearby metastable state on nucleation and the occurrence of multistep nucleation processes have been studied in the crystallization of a range of systems including colloids [10,11], proteins [12], and patchy particles [13], and in the crystallization of molecular solids from solution [14].In contrast, the kinetic processes of solid-solid phase transitions, which involve complex structural rearrangements [15], have received considerably less attention [16]. Solid-solid transitions usually occur in a martensitic fashion [17,18] involving the concerted, diffusionless motion of the atoms in the unit cell. Anisotropic stress, rapid quenching, and a small system size have been found to promote martensitic transformations [19]. 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 alternati...

A metastable liquid melted from a crystalline solid under decompression

“…In contrast, a crystalline solid always melts into a liquid above the melting line3, although the melting process may be affected by factors such as heating rate, impurities, particle size and shear stress. Recently, there has been a growing interest4567891011 in studying whether a crystalline solid may directly melt into a metastable liquid below melting line ( T m ) and the mechanisms underlying it.…”

“…proposed a transient melting process that occurs in a solid–solid phase transformation at temperatures significantly below the melting temperature due to internal stress. Recently such transient liquid has been experimentally observed using polymeric colloidal systems678, where the particles mimic atoms. A two-step diffusive nucleation pathway was observed in a structural transition, that is, appearance of transient liquid first followed by its crystallization6.…”

Revealing Three-Dimensional Structure of an Individual Colloidal Crystal Grain by Coherent X-Ray Diffractive Imaging

“…As a result, an exact stacking sequence of hexagonal close-packed layers including planar and linear defects were identified. Colloidal crystals nowadays are actively exploited as an important model system to study nucleation phenomena in freezing, melting, and solid-solid phase transitions [1][2][3][4][5], jamming and glass formation [6,7]. In addition, colloidal crystals are attractive for multiple applications since they can be used as large-scale templates to fabricate novel materials with unique optical properties such as the full photonic band gap, "slow" photons and negative refraction, as well as materials for application in catalysis, biomaterials, and sensorics [8][9][10][11].…”

Diverging Time Scale in the Dimensional Crossover for Liquids in Strong Confinement

“…There, the simplest geometry to investigate the effects of strong confinement is a slit where fluid particles are restricted to a narrow space between two smooth parallel plates, but also tubes or spherical confinements have been realized experimentally [24][25][26]. Computer simulations and experiments for the planar confinement have revealed an exotic equilibrium phase behavior due to commensurable stacking [27][28][29][30][31][32][33][34] as well as the hexatic phases in the limit of quasi-2D confinement [35,36]. Confinement induced order-disorder phase transitions for certain nonpolar liquids have also been reported in several experiments [37], but the interpretation has been challenged in favor of a glass transition [38][39][40].…”