Phase coexistence and high electrical properties in (KxNa0.96-xLi0.04)(Nb0.85Ta0.15)O3 piezoelectric ceramics

(KxNa0.96-xLi0.04)(Nb0.85Ta0.15)O-3 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 v(1) and a triply degenerate symmetric O-Nb-O bending vibration v(5) are detected as relatively strong scattering in the Raman spectra. The peak shifts of v(5) and v(1) 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 (d(33)=291 pC/N, k(p)=0.54, epsilon(r)=1167, tan delta=0.018, To-t=35 degrees C, T-C=351 degrees C, P-r=27.65 mu C/cm(2), and E-c=8.63 kV/cm). The results show that the equal K/Na ratio is not an essential condition in obtaining optimized electrical properties in (KxNa0.96-xLi0.04)(Nb0.85Ta0.15)O-3 ceramics.