The influence of order on the nucleation barrier

While it is conventionally thought that liquids turn to solids at their "freezing" point, it is often the case that materials must be supercooled (cooled below the freezing temperature) before nucleation actually happens. There is growing evidence that coupled processes, including chemical-structural ordering and orientational-translational ordering, are among the factors that affect exactly when and how nucleation occurs in liquids and crystals. Recent density functional calculations have demonstrated that such coupled routes, which are not incorporated within the one-dimensional framework of the classical theory, can dramatically influence the overall nucleation process. Here, some recently observed cases in metal alloys are discussed, establishing a relationship between developing order in undercooled liquids and the nucleation barrier, the influence of magnetic ordering on nucleation in Co-based melts, and the role of interfacial structure and chemistry on the catalytic efficiency of inoculants for heterogeneous nucleation.