Nucleation plays a critical role in many physical and biological phenomena, ranging from the formation of clouds to the initiation of neurodegenerative diseases. However, nucleation is a challenging process to study, especially in its early stages.

Here, the DMREF team advanced atomic electron tomography to study early-stage nucleation in 4D at atomic resolution. Three experimental observations contradict classical nucleation theory (CNT):

• Every nucleus has a core of one to a few atoms with the maximum order parameter and the order parameter gradient points from the core to the boundary of the nucleus.

• Nuclei undergo growth, fluctuation, dissolution, merging and/or division, which are regulated by the order parameter and its gradient.

• Early stage nuclei are anisotropic.

The results show that a theory beyond CNT is needed to describe nucleation at the atomic scale.