Achiral metastable crystals of sodium chlorate forming prior to chiral crystals in solution growth

Chiral symmetry breaking in NaClO₃ crystallization from an aqueous solution with perturbations has been of great interest. To understand the mechanism, several models focusing on the early stage of the crystallization have been proposed. However, they are ambiguous because the early stage has been barely explored directly. Here, we investigate the early stages of the crystallization process driven by droplet evaporation using a combination of direct in situ microscopic observations and cryogenic single-crystal XRD experiments. We demonstrate that an achiral crystal having P2₁/a symmetry, which is newly discovered for a solution growth, first appears in the droplet and then transforms into the chiral crystals. Additionally, determination of the lattice constants by XRD experiments (a = 8.42 Å, b = 5.26 Å, c = 6.70 Å, β = 109.71) revealed that the achiral phase should be identical to Phase III (a = 8.78 Å, b = 5.17 Å, c = 6.83 Å, β = 110), which is a high-temperature phase from a melt growth of NaClO₃. We advocate further assessment of the achiral crystal and a new pathway for the formation of chiral crystals via crystalline phase transition from achiral Phase III.