Space charge effect on the ferroelectric properties of a (K _xNa _1-x) ₂(Sr _yBa _1-y) ₄Nb ₁₀O ₃₀ solid-solution system

The influence of space charge on the ferroelectric properties of (K _xNa _1-x) ₂(Sr _yBa _1-y) ₄Nb ₁₀O ₃₀ (KNSBN) ceramics has been investigated in the temperature range between 30°C and 800°C for samples aged at room temperature and poled at temperatures above the Curie point (T _c ∼ 208° C for the composition x = 0.50, y = 0.75). In addition to a normal dielectric peak at T _c, KNSBN ceramics show a broad dielectric peak above T _c. This anomalous behavior above T _c can be explained by the space-charge polarization at the interfaces (grain boundaries, surfaces) due to mobile oxygen vacancies formed during the sintering process at high temperatures (∼ 1300°C). The experimental results are discussed in connection with the aging mechanisms. The analyses of the data suggest that the sample poled at temperatures below T _c is stabilized by the space-charge polarization at the grain boundaries which arises from the interaction between the polarity of the domain and the polarity of the space charge.