As a class of solid solution material, K(Ta,Nb)O₃ (KTN) single crystals have attracted significant interest due to their excellent piezoelectric and electro-optic performance. In this study, the piezoelectric properties of KTN were improved through Cu doping. Utilizing the advantages of composition-regulating phase transition, a large, high-quality Cu:KTN crystal measuring 30 mm × 25 mm × 20 mm in size was grown using the improved top seeded solution growth method. Cu-doped KTN exhibited better piezoelectric properties than pure KTN, and the full matrix parameters were investigated. Excellent dielectric, piezoelectric, and electromechanical coupling responses (ε₁₁^T~2,136, d₃₃~303 pC/N, k_t~0.515, and k₃₃~0.672) were successfully obtained. To realize the optimized orientation of the piezoelectric properties, the orientation dependence of the single domain properties was investigated, and the mechanism of Cu doping to improve piezoelectric properties was explored. We found that 0.36% (in mass) Cu doped crystal consisted of A-site substitution, following ferroelectric and domain analysis. The small ferroelectric domain sizes led to a large domain wall mobility rate and high domain wall density, which contributed to high piezoelectric properties. This work revealed the effect of A-position doping on piezoelectric properties and provided a theoretical and experimental foundation for the performance optimization of KTN-based materials.

Electro-optic (EO) crystals are important material for all-solid-state laser technology, which can be used to fabricate various laser modulators, such as EO switches, laser deflectors, and optical waveguide. The improvements in new high-efficiency EO crystal materials have held great significance to the development of laser technology. Potassium tantalate niobate (KTN) is a popular multifunctional crystal because of its remarkable and excellent quadratic EO effect. KTN EO modulation technology offers numerous advantages, such as high efficiency, good stability, a quick response time, and inertia-free characteristics. In this paper, we summarize the research progress of KTN series crystals systemically, including the theoretical exploration on quadratic EO effect, solid-melt crystal growth technique, comprehensive physical characterization, new physical effect and mechanisms exploration, new EO devices development and design. The EO modulation technique based on the Kerr effect of KTN series crystal offers obvious advantages in reducing the drive voltage and device size, which could better meet the developmental needs of future lasers with a wide wavelength, miniaturization, and integration. This may provide theoretical guidance and an experimental basis for the design and development of new EO crystal devices and promote the development of laser technology.

Perovskite K(Ta, Nb)O₃ (KTN) single crystal has drawn great interests for its outstanding electro-optic performance and excellent piezoelectric response. However, growth of compositionally uniform KTN single crystals has always been a great challenge for the great segregation difference between Nb and Ta. In this work, we propose a thermal field optimization strategy to resolve this challenge. Homogenous Sn doped KTN (Sn: KTN) single crystal with significantly reduced composition gradient (0.003 mol/mm, 1/4–1/8 of other KTN system), minimal T_C variation (13 ℃) and excellent piezoelectric and dielectric response (d₃₃ = 373 pC/N and ε₃₃^T = 5206) has been successfully achieved. We found that the functional properties of Sn: KTN were greatly affected by the near-room temperature tetragonal-cubic phase transition. From the intrinsic aspect, longitudinal lattice deformation becomes much easier, resulting in maximum piezoelectric (d₃₃^*), dielectric (ε₃₃^T*), elastic (s₃₃^E*)and electromechanical coupling (k₃₃^*) coefficients along polar direction [001]_C. From the extrinsic aspect, both domain wall density and domain wall mobility are greatly improved for the reduced lattice distortion, which also contribute a lot to the functional properties. This work provides a simple and practical route for designing and growing high quality crystals, and more importantly, reveals the fundamental mechanism of the phase transitions/boundaries on the functional properties.