Reversible Kirkwood–Alder Transition Observed in Pt<sub>3</sub>Cu<sub>2</sub> Nanoctahedron Assemblies under Controlled Solvent Annealing/Drying Conditions

We report a Kirkwood–Alder transition in a system of nonspherical Pt<sub>3</sub>Cu<sub>2</sub> nanoctahedra coated with oleic acid and oleylamine ligands. Using both transmission electron microscopy tomography with 3D reconstruction analysis and synchrotron-based <i>in-situ</i> grazing-incidence small-angle X-ray scattering (GISAXS) techniques, we specifically determined that these nanoctahedra can assemble into an open structure in which the nanoctahedra are arranged tip-to-tip to form a <i>bcc</i> superlattice with a low packing efficiency. Using <i>in-situ</i> and real-time GISAXS, we further observed a “nanoctahedron crystallization” as a soft Kirkwood–Alder transition, that is, the soft nanoactahedra crystallize at a critical concentration and possess continuous crystalline states during a period of solvent evaporation. Finally, we found a reversible change of the superlattice constant during the solvent annealing and evaporation/drying processes.