Recently, the research team of Prof. Richeng Yu and Dr. Xi Shen published a manuscript entitled “In situ TEM analysis of reversible non-180° domain switching in (K,Na)NbO₃ single crystals” in Journal of Materials Chemistry A (DOI: 10.1039/d3ta00737e).

Piezoelectric materials have the function of converting mechanical energy into electrical energy or vice versa, so they are widely used in ultrasonic transducers and actuators, especially in fields of medical care, and industry. Reversible non-180° domain switching could enhance the electric field-induced strain of ferroelectrics. In this work, the reversible domain wall motion and the non-180° domain switching in compositionally graded KNN single crystals have been studied by an in situ transmission electron microscope (TEM) holder using electric field-available sample holder. We propose that the restoring force derives from the built-in flexoelectric effect caused by compositional fluctuation on an atomic scale.

 The main points are as follows：

 With increasing concern for human health and environmental protection, the application of lead-based materials is restricted by many regulations. One of the most promising substitutes for lead-based piezoelectric ceramics is potassium sodium niobite [(K,Na)NbO₃ (KNN)]. In recent years, the piezoelectric coefficient d₃₃ of KNN-based materials has increased rapidly. However, the strain performance of KNN-based piezoelectric material is improved much more slowly than its piezoelectric coefficient d₃₃. The slow progress of strain performances of KNN is caused by an inadequate understanding of the reversible non-180° domain switching. The discovery and study of reversible non-180 °domains in KNN-based piezoelectric material is of great significance for the design of high-performance materials.

Transmission electron microscopy (TEM) has obvious advantages in spatial resolution, among which scanning transmission electron microscopy (STEM) has sub-angstrom spatial resolution and can image atomic columns. In situ TEM observation of domain switching can be realized by transferring specimens to an electrical chip with the focused ion beam and applying voltage with an electric field TEM holder. Figure 1 shows a schematic diagram of an electric chip in TEM.

Fig. 1 The schematic diagram of an electric chip in a TEM

Fig. 2 The bright-field (BF) image and corresponding selected area electron diffraction (SAED)

 The bright field image shows the projection of domain area and orientation of the domain wall. Figure 2 shows five lamellar-shaped domains embedded in the matrix domain. Combined with SAED, the crystallographic orientation of the domain wall projection can be determined as [30], and it is inferred that the domain wall is 60° domain walls. Voltage is applied to the sample, and gradually increased from 0V to 24V. The area of the five lamellar-shaped domains increases significantly, which represents the switching of the matrix domain to the lamellar-shaped domains. When the voltage gradually decays from 24V to 0V, the ferroelectric domain walls move in reverse which indicates that the domain switching is reversible.

Fig. 3 TEM images of reversible 60° domain along [001]_C zone axis.

 The reversibility of the domain indicates the existence of an inherent restoring force in the material. The analyses of HAADF-STEM show the affluent K or Na atomic columns are randomly distributed, indicating the existence of compositional fluctuation on an atomic scale. The local compositional fluctuation leads to strain fluctuation on an atomic scale, the strains of some unit-cells may exceed |±2%|. The different value of strains between neighboring unit-cells can generate the strong flexoelectric effect. We suggest that the flexoelectric effect is the origin of restoring force in the crystals. The flexoelectric effect makes the initial polarization direction of the material in the lowest energy state. The flexoelectricity is caused by the fluctuation of the composition, which is unchangeable under the electric field. When the electric field is moved away, the polarization will rotate back to the initial polarization orientations.

Fig. 4 Analysis of HAADF-STEM for composition fluctuations, stain fluctuations, and polarization rotation.

 In summary, the relative content of K/Na in the A-site atomic column fluctuates, and the induced flexoelectric effect becomes the restoring force of ferroelectric domains in (K,Na)NbO₃ single crystals, causing the reversible non-180° domain switching.