The birth of a crystal could be far less clearly defined than the scenario described for decades in many textbooks.
A crystal is a solid whose atoms adopt a regular arrangement. Crystallization begins with tiny ordered regions, called nuclei, which can then grow. Classical theory generally describes these nuclei as small crystals separated from the disordered material by a fairly sharp boundary.
Example of the atomic structure of a crystal called "Zangenite".
Credit: Shihao Zang/NYU
To observe this stage directly, researchers led by the University of California, Los Angeles, studied nanoparticles of alloys containing several chemical elements. They used a microscopy technique capable of reconstructing the position of atoms in three dimensions. In total, the team determined the structure of 8,160 crystal nuclei.
The result does not correspond to a simple separation between a crystallized part and a disordered part. At the heart of a nucleus, the atoms are highly organized. Moving toward its edge, this order gradually decreases. The transition therefore looks more like a gradual zone than a precise boundary.
This observation changes how the very beginning of crystallization is described. The researchers propose a model in which the degree of organization varies within the nucleus itself. Classical theory then remains valid in the special case where the transition becomes very abrupt.
The study also reveals that the organization of atoms depends on their chemical nature. The observed alloys contain several mixed metals. The way certain elements cluster locally therefore accompanies the emergence of crystalline order. Structure and composition evolve together during this initial stage.
The nuclei do not grow only by adding atoms. Most of those observed can also merge with neighboring nuclei whose orientation is similar. A minority join together differently and produce particular regions in the final crystal, called twins. These details can influence a material’s properties.
In practical terms, better describing this formation process could help explain how manufacturing modifies the final structure of metals and other solids. The alloys studied here belong to a family in which several elements are present in significant proportions. Some are sought after for their mechanical properties or their activity as catalysts.
It now remains to determine how broadly this model can be generalized. The authors believe that this gradual description may apply to many materials. New observations of other compositions will help determine whether the same progressive transition appears during crystallization.