Domain structure may greatly affect the dielectric and piezoelectric properties of a ferroelectric ceramic material. However, no investigations on the domain structure of poled (K,Na)NbO3-based ceramics have been reported so far. Since (K0.50Na0.50)NbO3 is the basic composition for the important type of lead-free piezoelectric materials, domain structure of the poled (K0.50Na0.50)NbO3 ceramics with the average grain size of about 8 μm were explored by observing the domain patterns with an acid etching technique in this study. It was found that domain patterns show usually either a single set of parallel domain stripes or a few sets of parallel domain stripes in polycrystalline grains. The average domain widths in different sets of parallel domain stripes vary largely from 150 nm to 750 nm. The intersection angels between two adjacent sets of domain stripes are around 45° or 135°. Two models of domain configuration were proposed to explain those domain patterns that consist of more than two sets of parallel domain stripes. One is built up in the way that the intersection angles between two adjacent sets of domain stripes are acute and consists of 90°-, 60°- and 120°-domain walls. The other one is formed in the way that the intersection angles between two adjacent sets of domain stripes are obtuse and is composed of 180°-, 90°- and 120°-domain walls. Additionally, the change of intersection angle between two adjacent sets of domain stripes with varying the observation plane is discussed.

The domain configuration of lead-free (K0.48Na0.52)(Nb0.96Sb0.04)O3-Bi0.50(Na0.82K0.18)0.50ZrO3 ceramics with rhombohedral-tetragonal morphotropic phase boundary, accounting for the high piezoelectric property and good thermal stability, were systematically studied. Short domain segments (before poling) and long domain stripes with wedge-shaped or furcated ends (after poling) were found to be typical domain configurations. The reduced elastic energy, lattice distortion, and internal stress, due to the coexistence of rhombohedral and tetragonal phases, result in much easier domain reorientation and domain wall motion, responsible for the high piezoelectric properties, being on the order of 460 pC/N, in which the extrinsic contribution from irreversible domain switching was estimated to be around 50% of the total piezoelectricity. Minor piezoelectric property variations (<6% over a temperature range from -50 to 100 °C) were observed as a function of temperature, showing a good thermal stability. In addition, nanodomains (50 ± 2 nm) were found to be assembled into domain stripes after poling, believed to benefit the high piezoelectric properties but not causing much thermal instability due to the small quantity.