Ultra-high piezoelectric properties and labyrinthine-domain structure in (K,Na)(Ta,Nb)O₃ with phase boundaries

(K,Na)NbO₃-based materials exhibit high potential for applications in electronic devices. Generally, to improve piezoelectric properties to meet the requirements of applications, a polymorphic phase transition boundary is required. However, because of the difficulty in growing potassium sodium niobate-based single crystals, with which the intrinsic characteristics of the material system can be best investigated, there are few studies on the origin of the high performance. Here, a KNTN single crystal with K/Na and Ta/Nb composition gradients was grown successfully via the top-seed solution growth method. Samples with compositions of K_0.41Na_0.59Ta_0.41Nb_0.59O₃ (KNTN41), K_0.49Na_0.51Ta_0.34Nb_0.66O₃ (KNTN34), and K_0.59Na_0.41Ta_0.28Nb_0.72O₃ (KNTN28) were cut along the (001)_PC direction. The KNTN41 sample exhibited an excellent piezoelectric coefficient (d₃₃ = 565 pC N⁻¹), which originated from the domain density observed via a polarized light microscope. In addition, the local domain patterns of the three samples observed via a piezoresponse force microscope exhibit different characteristics. In KNTN41 with a PPT boundary, the domain construction exhibits a labyrinthine-domain structure, whose high domain activity is the origin of its high piezoelectric properties.