As-grown domain structure and local switching in [111]-cut Cu-doped KNN single crystals

A comprehensive study of the ferroelectric domain structure in a [111]c-oriented KNN-Cu single crystal grown by the Bridgman route is presented. The research focused on both visualizing the domain structures in the as-grown crystal and the effects of local poling on them, which is crucial for understanding the microscopic behavior and its correlation with macroscopic piezoelectric properties. The study also reveals the sequential phase transformation from cubic to orthorhombic during cooling, observed optically. The resulting orthorhombic phase exhibits predominantly quasi-periodic lamellar domains with characteristic 60° domain walls and periods of 1–2 μm. By piezoresponse force microscopy, a complex as-grown domain structure consisting of typical herringbone, zigzag, and watermark patterns for KNN-based crystals was identified. Local manipulation with a biased SPM tip demonstrated the domain structure rearrangement and significant reduction in domain sizes. Step-by-step local switching revealed a complex domain structure with charged domain walls, where the created domain size obeys the linear law typical for ferroelectric crystals. The findings contribute to a deeper understanding of the material's microscopic behavior and its relationship with macroscopic piezoelectric properties, filling a gap in the visualization of domain structures in CuO-doped KNN crystals.