Hierarchical nucleation pathways are ubiquitous in the synthesis of minerals and materials. In the case of zeolites and metal-organic frameworks, preorganized multi-ion "secondary building units" (SBUs) have been proposed as fundamental building blocks. However, detailing the progress of multi-step reaction mechanisms from monomeric species to stable crystals and defining the structures of the SBUs remains an unmet challenge. Combining in situ nuclear magnetic resonance, small-angle X-ray scattering, and atomic force microscopy, we show that crystallization of the framework silicate, cyclosilicate hydrate, occurs through an assembly of cubic octameric Q 3 8 polyanions formed through cross-linking and polymerization of smaller silicate monomers and other oligomers. These Q 3 8 are stabilized by hydrogen bonds with surrounding H 2 O and tetramethylammonium ions (TMA + ). When Q 3 8 levels reach a threshold of � 32 % of the total silicate species, nucleation occurs. Further growth proceeds through the incorporation of [(TMA) x (Q 3 8 )•n H 2 O] (xÀ 8) clathrate complexes into step edges on the crystals.

Nucleation is the seminal event in crystallization and predicting its progression is a key challenge across diverse scientific fields. Although classical nucleation theory successfully describes some systems, it fails to do so in a growing list of materials due to free energetic and kinetic complexities1. In particular, when systems are driven far from equilibrium, the high chemical potential opens pathways that pass through metastable polymorphs, amorphous states, and even dense liquid phases that can form absent the energy barrier normally opposing nucleation1-5. CaCO3 provides a canonical example6-8; following discovery of the CaCO3 “polymer-induced liquid precursor”9,10, which forms upon introduction of poly-ionic polymers10-12 like polyacrylic acid13,14, numerous studies have concluded that even pure CaCO3 possesses a dense liquid phase. This phase — likely stabilized by poly-ionic proteins15-19 — is proposed to serve as a precursor to biomineral formation15, transforming first to the amorphous phase and then to crystalline products3,20-22. Yet little is known about the mechanisms of CaCO3 dense liquid formation and solidification, its composition is unknown, and its relationship to the polymer-induced liquid-precursor remains unclear. Using in situ methods, we show that this phase forms by spinodal decomposition, is a bicarbonate comprised of 1Ca2+:2HCO3-:7±2H2O, and transforms to hollow particles of hydrated amorphous CaCO3 with the release of CO2 and H2O. Acidic proteins and polymers extend liquid phase lifetimes but leave this chemical pathway unchanged. Molecular simulations suggest the liquid phase forms via direct condensation of solvated Ca2+•(HCO3-)2 complexes that react due to proximity effects in the confinement of the liquid droplets. The findings provide an in-depth picture of CaCO3 nucleation via liquid-liquid phase separation, elucidating an often-proposed pathway by which nature orchestrates the complex process of biomineralization.

Shape-controlled colloidal nanocrystal syntheses often require aid from facet-selective solution-phase chemical additives to regulate atom addition/migration fluxes or oriented particle attachment. Because of their highly tunable chemical property and robustness to a wide range of experimental conditions, peptoids contribute to a very promising group of next-generation functional chemical additives. To generalize the design philosophy, it is critical to understand the origin of facet selectivity at the molecular level. We employ molecular dynamics simulations and biased sampling methods to investigate the origin of Au(111)-favored adsorption of a peptoid, Nce3Ncp6, that is evidenced to assist the formation of five-fold twinned nanostructures. We find that the facet-selectivity is achieved through a synergistic effect of both molecule-surface and solvent-surface interactions. Extending beyond the single-chain scenario, the order of peptoid-peptoid and peptoid-surface energetics, i.e., peptoid-Au(100) < peptoid-peptoid < peptoid-Au(111), further amplifies the distinct behavior of Nce3Ncp6 chains on different Au surfaces. Our studies set the stage for future peptoid design in shape-controlled nanocrystal syntheses by probing the facet selectivity from various perspectives.

Hierarchical nucleation pathways are ubiquitous in the synthesis of minerals and materials. In the case of zeolites and metal–organic frameworks, pre‐organized multi‐ion “secondary building units” (SBUs) have been proposed as fundamental building blocks. However, detailing the progress of multi‐step reaction mechanisms from monomeric species to stable crystals and defining the structures of the SBUs remains an unmet challenge. Combining in situ nuclear magnetic resonance, small‐angle X‐ray scattering, and atomic force microscopy, we show that crystallization of the framework silicate, cyclosilicate hydrate, occurs through an assembly of cubic octameric Q38 polyanions formed through cross‐linking and polymerization of smaller silicate monomers and other oligomers. These Q38 are stabilized by hydrogen bonds with surrounding H2O and tetramethylammonium ions (TMA+). When Q38 levels reach a threshold of ≈32 % of the total silicate species, nucleation occurs. Further growth proceeds through the incorporation of [(TMA)x(Q38)⋅n H2O](x−8) clathrate complexes into step edges on the crystals.