2, Zhejiang University, Hangzhou, , China
3, Xiamen University, Xiamen, , China
Nucleation is the seminal process in crystal formation. While the classical picture of nucleation envisions addition of monomeric species to a growing nucleus that exhibits the structure of the bulk crystal, recent observations have revealed a rich set of hierarchical pathways involving higher-order species ranging from multi-ion complexes to dense liquid droplets that often form an initial transient crystalline or amorphous phase, which subsequently transforms to the final ordered phase. Our understanding of these multi-stage pathways and the factors that lead to their selection has been limited by availability of tools to investigate them in situ. Here we use liquid phase TEM to observe nucleation and transformation in the calcium carbonate and iron oxyhydroxide (FeOx) systems, both of which exhibit multiple polymorphs and are known for forming transient amorphous or disordered phases. For the calcium carbonate system, we used a series of additives — Mg, citrate and poly-acrylate — to modulate the transformation from amorphous calcium carbonate (ACC) to crystalline polymorphs. For citrate, PAA and low Mg content (≤2.5 mM), we find dissolution/re-precipitation dominates, with each additive extending the ACC lifetime and/or dissolution timescale, but the final polymorphs always exhibit the expected crystal morphology. In contrast, for [Mg] ≥ 5 mM, transformation occurs via loss of structural water seen both via an increase in electron density and changes in the Raman spectrum in the absence of morphological changes, leading to spheroidal Mg-calcite. TGA shows Mg brings excess water into ACC and molecular dynamics simulations predict this promotes atomic rearrangement. We hypothesize that slow dehydration coupled with ease of reorganization enables the isomorphic conversion. For the FeOx system we focused on crystallization of spindle-shaped hematite (Fe2O3) from ferrihydrite (5 Fe2O3-9H2O, fh). Our results show the spindles consist of atomically aligned domains organized into a second-order, hierarchical, rod-like structures penetrated by nm-scale pores. This structure forms as follows: First, the fh-to-hematite transformation is highly localized, with the initial hematite nucleating on fh surfaces. Second, spindles grow by addition of new hematite particles that nucleate from the solution. Whether the new particles form directly on existing hematite surfaces or in their immediate vicinity — after which they attach across the small remaining gap — is unclear. This process can even lead to formation of half-spindles with the growth direction pointing away from the mass of fh particles and into the fh-free bulk solution. Consequently, spindles do not form through fh attachment to hematite, nor even via random, independent hematite nucleation events followed by diffusion and attachment, but rather via an autocatalytic process in which fh provides a solute source, but the hematite interface drives secondary nucleation and attachment.