Stationary Atoms in Liquid Metals and Their Role in Solidification Mechanisms

According to common understanding, the primary difference between a liquid and a solid metal lies in atomic motion─atoms move rapidly in liquids, while they remain stationary in a solid lattice. The solidification process involves a transition from random atomic motion to an ordered crystalline structure, with nucleation playing a crucial role. However, our research indicates that the boundary between these two phases is not as distinct as previously believed: liquid metal nanoparticles can contain stationary atoms, and the number and positions of these atoms influence the solidification pathway upon cooling. Using spherical and chromatic aberration-corrected high-resolution transmission electron microscopy (HRTEM) at low accelerating voltages, we studied the solidification of platinum, palladium, and gold. We have developed a methodology that enables imaging of metal particles over a wide temperature range, from 20 to 800 °C, without compromising atomic resolution. When a nanoparticle melts, the contrast contribution of the fast-moving atoms vanishes in the HRTEM images, allowing stationary atoms to be visualized through the liquid layer as distinct atomic points of contrast that remain fixed in position on the imaging time scale (1 s or longer). These atoms are pinned at vacancy defect sites on graphene. By conducting HRTEM image contrast analysis during time-series imaging of individual 3-6 nm particles while changing the temperature from 800 to 20 °C, we uncover the mechanisms behind classical crystal nucleation, amorphous solidification, and the formation of supercooled liquid platinum. If the number of stationary platinum atoms is small (approximately fewer than 10) and positioned randomly, liquid-to-crystal nucleation can occur. However, if the number is higher, these stationary atoms can disrupt the crystallization process, particularly if they align along the perimeter of the liquid nanoparticle. We found that liquid nanodroplets, corralled by stationary atoms, remain liquid down to 200-300 °C, which is several hundred degrees below the bulk metal crystallization temperature. In these cases, supercooled liquid metal transforms into a metastable amorphous solid instead of crystallizing. Our results highlight the significance of stationary atoms in liquids, influenced by the local environment, which may hold significant implications for the use of metal nanoparticles on carbon in heterogeneous catalysis and other thermally activated processes.