Defect Migration and Phase Transformations in Two-Dimensional Iron Chloride inside Bilayer Graphene

The intercalation of metal chlorides, and particularly iron chlorides, into graphitic carbon structures has recently received lots of attention, as it can not only protect this two-dimensional (2D) magnetic system from the effects of the environment but also substantially alter the magnetic, electronic, and optical properties of both the intercalant and host material. At the same time, intercalation can result in the formation of structural defects or defects can appear under external stimuli, which can affect materials performance. These aspects have received so far little attention in dedicated experiments. In this study, we investigate the behavior of atomic-scale defects in iron chlorides intercalated into bilayer graphene by using scanning transmission electron microscopy and first-principles calculations. We observe transformations between the FeCl₂ and FeCl₃ phases and elucidate the role of defects in the transformations. Specifically, three types of defects are identified: Fe vacancies in FeCl₂ domains and Fe adatoms and interstitials in FeCl₃ domains, each exhibiting distinct dynamic behaviors. We also observed a crystalline phase with an unusual stoichiometry of Fe₅Cl₁₈ that has not been reported before. Our findings not only advance the understanding of intercalation mechanism of 2D materials but also highlight the profound impact of atomic-scale defects on their properties and potential technological applications.