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Abstract Solute structure and its evolution in supersaturated aqueous solutions are key clues to understand Ostwald’s step rule. Here, we measure the structural evolution of solute molecules in highly supersaturated solutions of KH₂PO₄(KDP) and NH₄H₂PO₄(ADP) using a combination of electrostatic levitation and synchrotron X-ray scattering. The measurement reveals the existence of a solution-solution transition in KDP solution, caused by changing molecular symmetries and structural evolution of the solution with supersaturation. Moreover, we find that the molecular symmetry of H₂PO₄^-impacts on phase selection. These findings manifest that molecular symmetry and its structural evolution can govern the crystallization pathways in aqueous solutions, explaining the microscopic origin of Ostwald’s step rule. 

For more than a century, Classical Nucleation Theory (CNT) has been used to explain the process of crystallization in supersaturated solutions. According to CNT, nucleation is a single-step process that occurs via monomer-by-monomer addition. However, recent findings from experiments and numerical simulations have shown that nucleation is a multi-step process that occurs via more complex pathways that involve intermediate species such as ion complexes, dense liquid precursors, or even nanocrystals. Such non-classical pathways observed in protein solutions, colloidal suspensions and electrolytes are reviewed in this paper. The formation of stable Pre-nucleation Clusters (PNCs) in the crystallization of biominerals is also discussed. In spite of the mounting evidence for non-classical nucleation behaviors, the knowledge about the structural evolution of the intermediate phases and their role in polymorph selection is still limited. It has also been observed that gravitational force interferes with the crystallization behavior of materials thereby posing limitation to ground-based experiments. Microgravity conditions, coupled with containerless processing techniques provide a suitable alternative to overcome these limitations.