  The fundamental understanding of crystallization, in terms of microscopic
kinetic and thermodynamic details, remains a key challenge in the physical
sciences. Here, by using in situ graphene liquid cell transmission electron
microscopy, we reveal the atomistic mechanism of NaCl crystallization from
solutions confined within graphene cells. We find that rock salt NaCl forms
with a peculiar hexagonal morphology. We also see the emergence of a transitory
graphite-like phase, which may act as an intermediate in a two-step pathway.
With the aid of density functional theory calculations, we propose that these
observations result from a delicate balance between the substrate-solute
interaction and thermodynamics under confinement. Our results highlight the
impact of confinement on both the kinetics and thermodynamics of
crystallization, offering new insights into heterogeneous crystallization
theory and a potential avenue for materials design.
