Discovery of a liquid crystal phase of sodium halides via a nonclassical nucleation pathway

The crystallization of ionic crystals has traditionally been explained by Gibbs's classical nucleation theory. However, recent observations of intermediate phases during nucleation suggest that the process may be more complex, necessitating new theoretical frameworks, though key empirical evidence remains elusive. In this study, we used microdroplets to investigate the crystallization of sodium halides (NaCl, NaBr, and NaI) under homogeneous nucleation conditions across a wide range of supersaturations. In the evaporating droplet, NaCl follows the classical nucleation pathway, whereas NaBr and NaI exhibit the formation of an intermediate phase prior to the nucleation of anhydrous and hydrous single crystals, respectively. Optical and computational analyses indicate that these intermediate phases are liquid crystal phases composed of contact ion pairs. These findings establish a new theoretical framework for crystal nucleation and growth and offer methods to control nucleation pathways, enabling us to achieve desired crystals regardless of specific conditions. This research identifies a previously unknown liquid crystal intermediate phase during sodium halide crystallization, revealing a nonclassical nucleation pathway. Understanding these intermediate phases composed of contact ion pairs opens new opportunities to design materials with tailored crystal structures, impacting fields such as pharmaceuticals and advanced material design.