Rhombohedral-tetragonal phase coexistence and piezoelectric properties based on potassium-sodium niobate ternary system

In this work, the rhombohedral-tetragonal (R-T) phase coexistence has been designed using the ternary system consisting of (0.99-x)K_0.48Na _0.52NbO₃-0.01B_0.5Na_0.5TiO ₃ xBi_0.5(Na_0.7K_0.2Li _0.1)_0.5ZrO₃. The R-T phase coexistence zone was identified in the ceramics with 0.03 ≤ x < 0.05, as confirmed by the phase diagram derived from both temperature-dependent dielectric constant and the X-ray diffraction patterns. The relationship between R and T phase boundary and their electrical properties is investigated. For the compositions near the R-T phase boundary, the ceramics show the enhanced dielectric, ferroelectric, and piezoelectric properties. The ceramic with x = 0.035 has an optimum electrical behavior (e.g., d₃₃ ∼ 310 pC/N, k_p ∼ 0.45, P_r ∼ 23.4 μC/cm², E_C ∼ 17.4 kV/cm, ε_r ∼ 1249, tan δ ∼ 0.025, and T_C ∼ 338 °C). In addition, the good temperature and thermal stability of ferroelectric and piezoelectric properties were shown in the ceramic with x = 0.035. Underlying physical mechanisms for enhanced electrical properties were addressed. As a result, the design of ternary system can effectively promote the piezoelectricity of potassium-sodium niobate materials.