Effect of bismuth deficiency with K substitution for Na on piezoelectric properties and crystal structure of Bi0.5-x(Na0.8-yK0.2+y)0.5TiO3-3x/2 (0 ≤ x ≤ 0.015, 0 ≤ y ≤ 0.1) ceramics

The piezoelectric properties and crystal structure of Bi0.5- x(Na0.8-yK0.2+y)(0.5)TiO3-3x/2 (0 <= x <= 0.015, 0 <= y <= 0.1) ceramics were characterized in this study. The crystal-structure refinement for Bi0.5- x(Na0.8-yK0.2+y)(0.5)TiO3-3x/2 was carried out using synchrotron X-ray diffraction data and Rietveld analysis and revealed the coexistence of rhombohedral (R3c) and tetragonal (P4mm) phases for the compositions with 0 <= x <= 0.015, although the fraction of these phases was almost independent of the composition x (49% for the rhombohedral phase and 51% for the tetragonal phase at x = 0.015). The Bi deficiency led to an increase in the lattice distortion of rhombohedral phase and a decrease in the that of tetragonal phase. The piezoelectric constant (d(33)) value decreased from 151.5 to 120.6 pC/N as x was increased from 0 to 0.015, and a shift of the depolarization temperature to higher temperatures was also confirmed by in situ d(33) measurements. These results are attributed to an increase in the lattice distortion of rhombohedral phase enhanced by the Bi deficiency. The K substitution for Na in Bi-0.485(Na0.8-yK0.2+y)(0.5)TiO2.9775 led to the formation of a morphotropic phase boundary (MPB) in the composition range 0 <= y <= 0.06, resulting in an increase in the fraction of the tetragonal phase. Although a decrease in the mechanical quality factor (Q(m)) was observed for K-rich compositions, the highest d(33) value of 195.4 pC/N with a mechanical coupling factor of 32.9% was obtained for the Bi-0.485(Na0.76K0.24)(0.5)TiO2.9775 (y = 0.04) ceramic because of the enhancement of the tetragonality in the MPB region, resulting in the onset of depolarization at similar to 170 degrees C.