Geometrically driven reversible solid-liquid phase transition at the atomic scale.

Atomic-resolution observation of the liquid-solid phase transition within a geometrically confined nanocluster provides fundamental insights into heterogeneous nucleation mechanisms. In this work, using in situ transmission electron microscopy, we directly control and observe a single critical-sized bismuth nanocluster within a tunable nanoscale gap, driving it through a reversible cycle from quasi-amorphous nanodisc, to crystalline nanowire, to liquid nanodroplet. The cluster's aspect ratio, rather than its volume, is the primary descriptor governing these phase transitions, determined by the interplay between intrinsic surface anisotropy and interfacial energetics. Confinement also imposes texture, forcing the nanowire to adopt a preferred [Formula: see text] orientation that is absent in unconfined nanoparticles. These results provide the mechanistic foundation for geometry-driven phase and orientation selection, which enables the rational design of nanomaterials through engineered confinement.