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Effects of oxygen vacancies on the dielectric, electrical, and ferroelectric properties of Ba4Nd2Fe2Nb8O30 ceramics were investigated. A dielectric relaxation above Tc can be ascribed to the trap-controlled ac conduction around doubly ionized oxygen vacancies. The dc conductivity of the N2-annealed and O2-annealed samples is attributed to the long-range motion of the ..., and that of the as-sintered sample is considered to be governed by the electronic and oxygen-vacancy ionic mixed conduction mechanism. Low concentration and random distributed oxygen vacancies are propitious to the domain switching, while high concentration and allied oxygen defects hinder the domain-wall movement. [ABSTRACT FROM AUTHOR]