Nucleation of Solid Solutions and the Pivotal Role of Ordered Precursors

Crystallization generally proceeds through successive stages that lead to a gradual increase in organization. Using molecular simulation, we identify a novel nucleation pathway for solid solutions. We show that nucleation can proceed through the formation of a chemically ordered intermediate that is more organized than the end product. In the case of metallic copper-gold alloys, we unravel a new nucleation mechanism that relies on the formation of an ordered L12 precursor and can be summarized as follows: ($\text{liquid}\to L{1}_2$ $\text{precursor}\to \text{solid}$ solution). This novel nucleation pathway results from the low formation energy of $L{1}_2$ clusters. Such clusters then promote crystal nucleation. We also determine that this mechanism is composition-dependent by showing that the high formation energy of other ordered phases prevents them from acting as precursors of the nucleation process.