Preparation and dielectric properties of Ba4RFe0.5Nb9.5O30 (R = La, Nd, Eu, Gd) unfilled tungsten bronze ceramics

Lead-free relaxor ferroelectrics with the general structural formula Ba₄RFe_0.5Nb_9.5O₃₀ (R = La, Nd, Eu, Gd) (BRFN) were prepared using a standard solid-state sintering route, and the influence of the A site ion occupation on their electrical properties and dielectric characteristics was systematically discussed based on their structure. All compounds presented tetragonal tungsten bronze (TTB) structure with space group P4bm, where the A1 sites were exclusively occupied by R³⁺ ions and the A2 sites were exclusively occupied by Ba²⁺ ions, while the Fe³⁺ and Nb⁵⁺ ions were randomly distributed between the B1 and B2 sites. Furthermore, frequency and temperature caused BRFNs to present a broad permittivity peak accompanied by wide frequency dispersion. The maximum dielectric constant temperature gradually increased and the frequency dispersion gradually decreased as the A1-site ion size decreased. Complex impedance spectroscopy indicated that only the grain boundaries contributed to the conductive behavior of BRFN ceramics at high temperature, and the activation energy increased as the A1-site ion size decreased. The direct current (DC) conductivity of all compounds originated from the electrons generated by the second ionization of the oxygen vacancies and might have been caused by the hybrid conduction mechanism of the oxygen-vacancy ions and electrons. Raman spectra of the BRFN ceramics mostly contained multi-component and broad bands related to the internal vibrations of the octahedral NbO₆, and as the size of the A1-site ion decreased, the Nb-O stretching changed.