Dielectric, ferroelectric and impedance properties of Li⁺-doped 0.97Na_0.4K_0.1Bi_0.5TiO₃–0.03Ba_0.7Sr_0.3TiO₃ ceramics

A series of Li⁺-doped 0.97Na_0.4K_0.1Bi_0.5TiO₃–0.03Ba_0.7Sr_0.3TiO₃ ceramics were prepared by using the conventional solid-state reaction technique. The phase structure, dielectric, ferroelectric and impedance properties were systemically studied. X-ray powder diffraction patterns reveal that all the ceramics possess a pure perovskite phase. Ferroelectric property tests clearly show the disruption of the long-range ferroelectric order as the Li⁺ doping content increases. Dielectric permittivity results suggest that the samples experience a transformation from ferroelectric to relaxor phase with increasing the Li⁺ concentration, while the diffuse factors further confirm the enhanced relaxor characteristic. Complex ac impedance data shows that charge carriers obey the long-range conductivity mechanism at low doping levels (x ≤ 0.01), while it changes to a localized mechanism at high doping levels (x ≥ 0.03). The active energy fitted by the relaxation time becomes smaller as the Li⁺ concentration increases, revealing the increasing concentration of oxygen vacancies. We argue that the increasing concentration of vacancies should be responsible for the enhanced relaxor characteristic by disturbing the long-range ferroelectric orders.