In situ TEM observation of nanodomain mechanics in barium titanate under external loads

In ferroelectrics, domain walls have significant effects on ferroic properties. The mechanical behavior of domain walls has been investigated because of its scientific and technological importance. Numerous experiments and simulations have examined domain walls and their motion under mechanical strain. However, since nano- and micromechanical testing is challenging, previous studies have mainly involved indirect or intermittent observations, and there have been few real-time in situ observations. As a result, the mechanism by which mechanical loads induce wall motion remains elusive. Here, we directly observe stress-induced domain wall motion in real time by using rationally designed specimens and transmission electron microscopy (TEM). We imaged the domain walls directly as they moved and disappeared depending on the mechanical conditions during the experiments. Furthermore, we experimentally determined the mechanical criteria for wall motion. Our results not only provide fundamental knowledge about ferroelectrics but also provide information on how to control their dynamics from an engineering standpoint.

Figure 1

(a) Schematics of the designed tensile and bending tests. (a1) Tensile test: opening displacement $\delta$ is applied by inserting the tip, and the thinned part undergoes tension. (a2) Bending test: on pushing the tip, the loading point is displaced by $u_x$ and $u_y$, and the thinned part undergoes bending. (b) SEM and TEM dark-field images of specimens A, B, and C after fabrication and annealing. The diffraction patterns indicate the selected diffraction spots.

Figure 2

(a) TEM dark-field images and schematics of specimen A under loading. We calculate the global strain ${\epsilon }_{xx}^{\mathrm{global}}$ the thinned part undergoes by dividing the change in length at the top of the thinned part by the initial length. (b) Phase-field modeling and calculation of domain motion under tension. Contour color and arrow indicate the direction of polarization P.

Figure 3

(a) TEM dark-field images and schematics of specimen B under loading. (b) Phase-field modeling and calculation of domain motion under tension perpendicular to the wall.

Figure 4

(a) TEM dark-field images and schematics of specimen C under loading. (b) Phase-field modeling and calculation of domain motion induced by a bending strain, i.e., tension at the top and compression at the bottom.

Figure 5

Schematics of shear stress parallel to domain walls ${\tau }_{\mathrm{wall}}$ induced by loading. (a)–(c) Specimens A, B, and C, respectively. The dotted line indicates the wall. $\theta$ is the angle between the wall and the $y$ axis.