In Situ Probing of Early Stage Phase Transformation of KCl in Aqueous Solutions via Liquid-Cell TEM

Directly resolving the atomic-scale dynamics of phase transitions in alkali halide crystals after nucleation from saturated solutions remains challenging but is essential for understanding crystal growth. Here, in situ liquid-phase transmission electron microscopy (LP-TEM) was used to visualize the early stage structural evolution of KCl in aqueous solution. We observe the dissolution of [110]-oriented KCl domains followed by reprecipitation into [100]-oriented domains. Time-resolved radial distribution function (RDF) analysis derived from sequential fast Fourier transform (FFT) diffractograms quantitatively maps the kinetic pathway of this transformation. A previously unreported metastable hexagonal phase (h-KCl) is identified as a transient intermediate connecting two cubic domains. These results provide direct evidence for a cubic-hexagonal-cubic transformation pathway in a prototypical ionic crystal and reveal how nonequilibrium intermediates govern solution-mediated solid-state phase transitions.