Atomic-Scale Observation of Titanium-Ion Shifts in Barium Titanate Nanoparticles: Implications for Ferroelectric Applications

Ferroelectric nanoparticles (NPs) have attracted considerable attention owing to their size effect on the ferroelectricity and their possible application toward future electronic devices such as multilayer ceramic capacitors and ferroelectric random access memory. The ferroelectricity disappears for NPs smaller than the critical size, which has been an obstacle for the development of materials. Although the fundamental mechanisms of the size effect should be clarified to overcome this problem, the understanding has been made ambiguous by the fact that NPs of different morphologies prepared by different methods exhibit various critical sizes, which indicates that more investigations should be conducted on the appearance/disappearance of ferroelectricity in NPs. To gain insight into the appearance of ferroelectricity, atomic-scale characterizations are beneficial because the ferroelectricity is closely related to the atomistic structures. In the present study, atomic-scale scanning transmission electron microscopy (STEM) observations were conducted for a barium titanate NP prepared by a hydrothermal method, using a supercritical continuous-flow reaction system. Two STEM images were obtained with different foci: one was observed by focusing an electron probe on the top surface of the NP and the other on the middle. Different directions of titanium-ion shifts were observed near the top surface and in the middle of the NP, which could be explained by STEM image simulations using structural models with the presence of an additional region with a different titanium-ion-shift direction. The present findings imply that this NP should exhibit the ferroelectricity and contains two regions of different polarization directions.