Although non-classical crystallization pathways have been extensively studied in low-temperature systems (e.g. biomineralization), directly observing the dynamics of nucleation and growth under high-temperature and high-pressure magmatic conditions remains a major challenge. Using naturally step-quenched rock samples and high-resolution transmission electron microscopy, we obtained a series of nanoscale ‘snapshots’ of final state inversion crystallization process. Our results reveal that structural units in the melt first assemble into amorphous pre-nucleation clusters. Upon reaching a critical size (~3.5 nm in diameter), these clusters undergo a phase transition into crystal nuclei with a well-defined lattice. Subsequently, structural units migrate toward the crystal nuclei, forming a growth boundary layer at the crystal-melt interface. Within this layer, structural units assemble into growth units, which are integrated into the crystal lattice through a cooperative mechanism, driving crystal growth. These findings uncover a non-classical two-stage, four-step nucleation-growth pathway. Under this mechanism, densely distributed and nearly synchronously formed pre-nucleation clusters and crystal nuclei compete for limited nutrients and growth space, ultimately generating polycrystalline aggregates composed of numerous tiny crystals. Direct observation of crystal nucleation during magma crystallization demonstrates that the uniformly distributed phenocrysts in volcanic rocks and the randomly oriented crystals in near-equigranular intrusive rocks (e.g. granites) result from homogeneous nucleation. Notably, despite the vast environmental differences between high-temperature magmatic systems and low-temperature aqueous solutions, both follow the same energy-optimized crystallization pathway at the atomic scale, which remains unchanged regardless of external conditions (e.g. temperature, pressure, composition, viscosity) that influence crystal growth rates and morphologies. This appears to be a naturally designed, energy-minimizing pathway: pre-nucleation cluster → crystal nucleus → growth boundary layer → polycrystal (or single crystal).
High-entropy alloys combine multiple principal elements and can exhibit exceptional mechanical properties and catalytic activity. However, how they crystallize remains poorly understood because early nuclei are small, transient and chemically complex. Here we advance atomic electron tomography to determine the three-dimensional atomic structures and local chemical order of 8,160 high- and medium-entropy alloy nuclei. We find that nucleation proceeds through gradient ordering, in which structural order is highest at the core, decreases smoothly towards the boundary and is coupled to local chemical order. Most nuclei coalesce with nearly aligned crystal lattices, whereas a minority form twin boundaries. We develop the gradient nucleation pathways model, which generalizes classical nucleation theory by incorporating spatially varying structural order within each nucleus. The model captures diffuse, partially ordered nuclei, recovers classical nucleation theory in the sharp-interface limit and reveals multiple intermediate states. These results provide an atomistic framework for understanding crystal nucleation and growth across a broad range of materials.
Yakun Yuan, S. Moniri, Yao Yang et al.· Nature Materials· 0 citations
Nucleation is a key rate-limiting process in phase transition and phase separation. Recent studies highlight a significant discrepancy between experimentally measured nucleation rates and theoretical predictions, particularly when dynamic structural reordering occurs along multi-step pathways. To bridge this gap, we develop a multi-shell model with a space-time-dependent order-parameter field to describe the reordering-nucleation process, where structural reorganization couples with the early growth of condensed clusters. Through stochastic simulations, we track the time-resolved evolution of heterogeneous structural order inside growing clusters. Path analysis of the first-passage problem in early-stage nucleation demonstrates that shifting the reordering rate alters the nucleation rate by several orders of magnitude. Furthermore, as the coupling strength increases, the relationship between the mean first-passage time and reordering susceptibility shifts from monotonic to non-monotonic, exhibiting a turnover effect. We quantitatively rationalize these behaviors with an effective nucleation barrier that accounts for non-equilibrium properties. Our findings elucidate the mechanisms behind multi-step nucleation and offer a predictive framework for future studies.
Liesegang patterns, characterized by periodic precipitation, typically follow either a pre-nucleation or post-nucleation model. However, the factors governing the selection of either pathway in a single chemical system are poorly understood. In this study, we demonstrated a concentration-driven bifurcation between the pre-nucleation and post-nucleation models during the formation of fluorapatite (FAp) Liesegang patterns in an agarose matrix using two systematic sets of experimental conditions. First, a three-variable matrix systematically varying the concentrations of the three involved electrolytes (i.e., calcium chloride, phosphate buffer, and sodium fluoride) revealed a distinct difference between the pre- and post-nucleation pathways. Using time-lapse imaging, line profile analysis, and X-ray diffraction, we monitored the evolution of amorphous calcium phosphate (ACP) intermediates into other species such as crystalline FAp. At certain concentration conditions, the system followed a pre-nucleation model, in which discontinuous ACP growth and subsequent crystallization-induced shrinkage produced discrete bands. By contrast, other conditions triggered a post-nucleation pathway, in which a spatially continuous ACP phase first precipitated and then separated into discrete bands upon structural conversion. Second, by fixing the outer electrolyte (calcium chloride) concentration and systematically varying the two inner electrolyte concentrations (phosphate buffer and sodium fluoride), we successfully mapped the continuous crossover between these two regimes and identified a transition boundary where the features of both pathways co-existed. The study proposes that the bifurcation between the two models would be governed by the kinetic balance between the generation of metastable ACP and its structural conversion. These findings provide a broadly applicable framework for understanding pattern formation in mineral systems, which progress through metastable intermediates.
The graphite-to-diamond transition exhibits striking variability under high-pressure, high-temperature (HPHT) conditions, producing diamond, graphitic phases, or metastable, mixed diamond-graphite nanocomposites despite similar synthesis conditions. Existing atomistic models, largely based on idealised single-crystal graphite, do not explain the persistence of partially transformed intermediate states under HPHT conditions. Here, using large-scale molecular dynamics simulations, we show that precursor grain structure governs graphite-to-diamond transformation pathways by decoupling diamond nucleation from cooperative transformation propagation. Grain boundaries first facilitate local sp$^3$ nucleation, after which diamond growth propagates within individual grains but becomes arrested at crystallographically mismatched grain boundaries. As a result, structurally heterogeneous graphite stabilizes kinetically arrested mixed sp$^2$-sp$^3$ states, whereas large or single-crystalline domains favour cooperative bulk transformation into diamond. Our findings identify structural heterogeneity as a missing control parameter alongside pressure and temperature, reframing metastable transformation products as kinetically trapped states arising from precursor microstructure rather than thermodynamic intermediates. Precursor crystallinity therefore emerges as a practical control parameter governing graphite-to-diamond transformation pathways.
Zuzanna Malinowska-Trzmielak, Vilmos Neuman, M. Wilson· 0 citations
Gas–liquid interfacial nucleation can influence organic crystallization, yet its mechanistic role in polymorph selection remains poorly understood. Here, we demonstrate that nucleation at the gas–liquid interface of flufenamic acid solutions governs polymorph selection and gives rise to a pronounced concentration-dependent polymorphism, wherein low initial concentrations favor the nucleation of form I, higher concentrations yield form III, and intermediate concentrations selectively produce the metastable form IV. In situ synchrotron-based grazing-incidence wide-angle X-ray scattering (GIWAXS) directly resolves the formation and evolution of prenucleation assemblies at the interface, revealing three distinct interfacial molecular evolution pathways correlated with the emergence of specific polymorphs. Molecular dynamics (MD) simulations combined with energy calculations within the hybrid quantum mechanics/molecular mechanics (QM/MM) embedded-cluster framework further reveal interfacial enrichment, orientational bias, and conformer-dependent stabilization of stacking motifs, providing a microscopic interpretation of the experimentally observed selectivity. Together, these results establish the gas–liquid interface as an active structural selector that reshapes molecular organization prior to nucleation, offering a mechanistic framework for understanding and controlling polymorphism in evaporation-driven crystallization.
Yu Liu, Xu Zhang, Xingfan Zhang et al.· Journal of the American Chem...· 0 citations
Crystallization from solution is commonly described in terms of direct nucleation from dispersed monomeric species, yet this picture becomes inadequate when strong coordination or solvation suppresses free-particle formation. Here we resolve the growth pathway of Pt nanoparticles formed by CO-mediated reduction of Pt precursors using real-time mass spectrometry, infrared and UV-vis spectroscopy, ultrafast chemical exchange measurements, and theory. We show that classical monomer-based nucleation is bypassed: strong Pt-CO coordination stabilizes metastable multinuclear clusters, with [Pt3(CO)6]n2- (n = 4) emerging as a dominant intermediate. Rather than acting as seeds, these clusters undergo a collapse-reassembly process upon oxidation, in which partial ligand loss generates locally concentrated, coordinatively unsaturated Pt fragments that rapidly reorganize into crystalline nanoparticles, enabling growth without the entropic penalty of stochastic atom aggregation. This mechanism may be broadly relevant to nanoparticle formation under strong coordination and redox-active conditions, and may also extend to crystallization processes in strongly solvated systems like ions in aqueous solutions, where transient, non-equilibrium clusters mediate the transition from molecular precursors to extended crystalline matter.
