UCLA Researchers Image Crystal Formation Atom by Atom, Challenging Century-Old Theory

UCLA-led researchers have imaged crystal nucleation -- the process by which small, ordered regions called nuclei form inside disordered matter as a substance freezes, condenses or otherwise changes phase -- atom by atom in three dimensions. They found that crystals form with a gradual transition into the surrounding disordered material, rather than the sharp boundary predicted by a nearly century-old model called classical nucleation theory. The study was published in the journal Nature Materials.

The team, led by corresponding author Jianwei "John" Miao, a professor of physics and astronomy in the UCLA College and member of the California NanoSystems Institute at UCLA, examined nanoparticles made from high- and medium-entropy alloys, which combine several metallic elements in roughly equal amounts, unlike conventional alloys such as steel, dominated by one main element. The researchers heated the nanoparticles and then supercooled them from more than 3,000 degrees Fahrenheit to room temperature within a few hundredths of a second, trapping crystal nuclei at different stages of formation. Using atomic electron tomography, a 3D imaging technique that maps individual atoms, the team reconstructed more than 8,000 nuclei, ranging from fewer than 10 atoms to more than 1,000.

"The crystals were not uniform with a sharp boundary from the disordered atoms around them, as predicted by classical nucleation theory," Miao said. "Instead, we saw a gradient. Every nucleus had a core of highest crystallinity and then became more disordered as you go from that core to the boundary." To account for the pattern, the researchers developed a new equation called the gradient nucleation pathways model, which expands classical nucleation theory rather than discarding it -- substituting that case into the new equation reproduces the classical theory's own results, Miao said.

The researchers also found that most nearby crystal nuclei developed in the same orientation even before merging into larger crystals. The high- and medium-entropy alloys used in the study were first introduced about two decades ago.