In situ observation of phase transformation in iron carbide nanocrystals

This paper reports on the in situ observation of phase transformation in an iron carbide nanocrystal encapsulated in a graphitic shell by means of high resolution transmission electron microscopy (HR-TEM). A Fe₇C₃ nanocrystal in orthorhombic (o-Fe₇C₃) structure with carbon graphitic cover is captured at the initial time of the experiment. Under the projection of a high-energy electron beam (200kV), the graphitic carbon layer evaporates gradually and structural changes in orthorhombic (o-Fe₇C₃) crystal manifests simultaneously. Specifically, changes in crystal direction happens first and then the crystal structure switching between orthorhombic and hexagonal (h-Fe₇C₃) follows. Details analysis and conclusive evidences of the phase structure and transformation are presented and discussed. The appearance of o-Fe₇C₃ structure is captured for about 92min over 100min of observation, indicating the preference of o-Fe₇C₃ form over h-Fe₇C₃ form.