Two distinct dielectric relaxation mechanisms in the low-frequency range in (K_0.50Na_0.50) ₂(Sr_0.75Ba_0.25)₄Nb ₁₀O₃₀ ceramics

The effects of space charges on the ferroelectric properties of (K_0.50Na_0.50)₂(Sr_0.75Ba _0.25)₄Nb₁₀ O₃₀ ceramics were studied using the ac complex impedance technique. Using our sample, which was aged at room temperature in order to induce a motion of space charges, we observed two dielectric relaxation mechanisms, each having different physical characters in the low-frequency range at temperatures above the Curie point (T_c ∼208°C). One mechanism may be attributed to the hopping process of the space charges bound inside grains, while the other seems to be associated with the space charges at the grain boundary layers. The dielectric measurements were analyzed using equivalent circuits composed of a capacitor (1) connected in series and (2) connected in parallel with a parallel RC circuit. These results were also examined within the framework of a model based on space-charge migration, which was suggested as an essential process for aging. This leads to the conclusion that the two mechanisms are distinct.