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Solid-to-solid transitions usually occur via athermal nucleation pathways on pre-existing defects due to immense strain energy. However, the extent to which athermal nucleation persists under low strain energy comparable to the interface energy, and whether thermally-activated nucleation is still possible are mostly unknown. To address these questions, the microscopic observation of the transformation dynamics is a prerequisite. Using a charged colloidal system that allows the triggering of an fcc-to-bcc transition while enabling in-situ single-particle-level observation, we experimentally find both athermal and thermally-activated pathways controlled by the softness of the parent crystal. In particular, we reveal three new transition pathways: ingrain homogeneous nucleation driven by spontaneous dislocation generation, heterogeneous nucleation assisted by premelting grain boundaries, and wall-assisted growth. Our findings reveal the physical principles behind the system-dependent pathway selection and shed light on the control of solid-to-solid transitions through the parent phase’s softness and defect landscape.

Most systems have more than two stable crystalline states in the phase diagram, which is known as polymorphism. Crystallization in such a system is often under strong influence of competing orderings linked to those crystals. However, how such competition affects crystal nucleation and ordering toward the final crystalline state is largely unknown. This is primarily because the competition takes place locally and thus is masked by large positional fluctuations. We develop a unique method to correctly identify local symmetries by removing their distortions due to positional fluctuations. This allows us to experimentally access the spatiotemporal fluctuations of local symmetries at a single-particle level in crystallization of a charged colloidal system near the body-centered cubic–face-centered cubic border. Thus, we successfully reveal the crucial roles of competing ordering in the initial selection of polymorphs and the final grain boundary motion toward the most stable state from a microscopic perspective.

The dependence of magnetic transition on the treatment solution used in the preparation of magnetic nanoparticles was investigated using as-prepared products from paramagnetic FeOOH/Mg(OH)2via a chemically induced transition. Treatment using FeCl3and CuCl solutions led to a product that showed no magnetic transition, whereas the product after treatment with FeSO4or FeCl2solutions showed ferromagnetism. Experiments revealed that the magnetism was caused by the ferrimagneticγ-Fe2O3phase in the nanoparticles, which had a coating of ferric compound. This observation suggests that Fe2+in the treatment solution underwent oxidation to Fe3+, thereby inducing the magnetic transition. The magnetic nanoparticles prepared via treatment with an FeSO4solution contained a larger amount of the nonmagnetic phase. This resulted in weaker magnetization even though these nanoparticles were larger than those prepared by treatment with an FeCl2solution. The magnetic transition of the precursor (FeOOH/Mg(OH)2) was dependent upon treatment solutions and was essentially induced by the oxidation of Fe2+and simultaneous dehydration of FeOOH phase. The transition was independent of the acid radical ions in the treatment solution, but the coating on the magnetic crystallites varied with changes in the acid radical ion.