Phase coexistence and high electrical properties in (K_xNa _0.96-xLi0.04_0.04) (Nb_0.85Ta_0.15) O₃ piezoelectric ceramics

(K_xNa_0.96-xLi0.04_0.04 ) (Nb _0.85Ta_0.15 ) O₃ lead-free piezoelectric ceramics were produced by conventional solid-state reaction method. The effects of K/Na ratio on the phase transitional behavior, Raman spectrum, microstructure, and dielectric, piezoelectric, and ferroelectric properties of the ceramics have been investigated. The phase structure of the ceramics undergoes a transition from orthorhombic to tetragonal phase with increasing x. A double-degenerate symmetric O-Nb-O stretching vibration v1 and a triply degenerate symmetric O-Nb-O bending vibration v5 are detected as relatively strong scattering in the Raman spectra. The peak shifts of v5 and v1 modes all have a discontinuity with x between 0.42 and 0.46, which may suggest the coexistence of orthorhombic and tetragonal phases in this range. Properly modifying x reduces the sintering temperature, promotes the grain growth behavior, and improves the density of the ceramics. The polymorphic phase transition (at To-t) is shifted to near room temperature by increasing x to 0.44 (K/Na ratio of about 0.85:1), and the coexistence of orthorhombic and tetragonal phases in the ceramics at x=0.44 results in the optimized electrical properties (d33 =291 pC/N, kp =0.54, εr =1167, tan δ=0.018, To-t =35 °C, TC =351 °C, Pr =27.65 μC/ cm2, and Ec =8.63 kV/cm). The results show that the equal K/Na ratio is not an essential condition in obtaining optimized electrical properties in (K_xNa_0.96-xLi0.04 _0.04 ) (Nb_0.85Ta_0.15 ) O₃ ceramics.