Substances with the same chemical composition but different crystal structures are called crystal polymorphs. Because they have different physical and chemical properties, selectively growing the desired structure from among them is an important point in the creation of materials and medicines. However, the detailed process of crystallization accompanying the transition between polymorphs has not been elucidated, and a depiction at the molecular and atomic scale is required. In this study, they used a colloidal system as a model of phase transition to approach elucidating the mechanism of selection of crystal polymorphs.
The research group comprises Assistant Professor Jun Nozawa (at the time of the research) and Professor Kozo Fujiwara of the Institute for Materials Research, Tohoku University; Professor Masahide Sato of the Emerging Media Initiative, Kanazawa University; and Professor Satoshi Uda of the New Industry Creation Hatchery Center, Tohoku University. The formation of crystal polymorphs was realized through heteroepitaxial growth in colloidal crystals. The effects of polymorphic transitions on nucleation and crystal growth were elucidated by in situ observation with single-particle resolution. In particular, the study revealed that three types of polymorphic transitions occurring during nucleation and growth play an important role in the final selection of crystal polymorphs. The results of this research are expected to make a significant contribution to the control of crystal polymorphs across various systems, including drug discovery.
The results of this research will be published on April 9, 2025 (British Summer Time) in the scientific journal "Communications Physics" The article was published in the
【Presentation Points】
● Controlling crystal polymorphism (*1) is extremely important in various aspects of crystal growth, but the detailed mechanism by which crystal polymorph is selected has not yet been elucidated.
● In this study, the formation of colloidal crystal (*2) polymorphs, regular arrays of colloidal particles with diameters below one micrometer (submicron), was realized through heteroepitaxial growth (*3). In situ observations revealed nucleation (*4) and polymorphic transitions (*5) occurring during crystal growth.
● The research group clarified that the stability of the clusters before nucleation and the size of the crystals are important factors in inducing polymorphic transitions, and that these factors contribute to the selection of the final crystal polymorph. Furthermore, they demonstrated polymorph control focusing on the differences in cluster morphology in each polymorph.
Figure 1: (a) Microscopic images and schematic diagrams of the polymorphs (α phase: green, β phase: red) obtained by heteroepitaxial growth. Polystyrene particles with diameters of 860 nm were used as the epitaxial phase and 1300 nm as the substrate crystal. (b) Snapshots of the polymorphic transition during nucleation. (c) Snapshots of the solution-mediated β to α polymorphic transition occurring during crystal growth. (d) Snapshots of the solid α to solid β polymorphic transition occurring during crystal growth.
Figure 2: Schematic diagram showing various crystallization pathways by polymorphic transition.
【Glossary】
*1: Crystal polymorphism
Substances that have the same chemical formula but different crystal structures, such as graphite and diamond.
*2: Colloidal crystal
colloidal state refers to a condition in which submicron particles, larger than atoms or small molecules, are dispersed in a liquid; such particles are referred to as colloidal particles. A structure in which these particles are arranged in an ordered array, analogous to the arrangement of atoms or molecules in a crystal, is called a colloidal crystal. Colloidal crystals are used in applications as photonic materials and in research on phase transition mechanisms.
*3: Heteroepitaxial growth
Epitaxy is a growth mode in which a crystalline phase is grown on a crystalline substrate with a specific crystallographic orientation relationship. When the film and the substrate are of the same material, the process is referred to as homoepitaxy; when they differ, it is called heteroepitaxy.
*4: Nucleus formation
The process by which crystal seeds form in a gas or liquid phase due to thermal fluctuations.
*5: Polymorphic transition
The change from one polymorphic structure to another.
Click here to see the press release【Japanese only】
Journal: Communications Physics
Researcher's information: Masahide Sato
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