Frequency-temperature response of ferroelectromagnetic Pb(Fe 1/2Nb1/2)O3 ceramics obtained by different precursors. III. Dielectric relaxation near the transition temperature

Abstract

Dielectric relaxation processes occurring near the ferroelectric- paraelectric phase transition of ferroelectromagnetic Pb(Fe1/2Nb 1/2)O3 ceramics obtained by different precursors are discussed using microstructural and equivalent circuit modeling and the impedance spectroscopy technique. The frequency-temperature response was obtained from room temperature to 300 °C and from 20 Hz to 1 Mz. In correspondence with a previous structural, morphological, and temperature response study, appropriate microstructural, and equivalent circuit models were established. The frequency response study was carried out by means of the simultaneous analysis of the complex dielectric constant Ε̃ and admittance Ỹ functions and the dielectric loss, tan δ A strong absorption near the transition temperature region at a frequency around 1 MHz is discussed and is attributed with relaxation processes associated with domain reorientation, domain wall (DW) motion, and the dipolar behavior of ferroelectric materials. Such processes were found to take place inside the grain, and their low characteristic frequencies are explained by clamping effects of the DW due to the thermally activated diffusion of oxygen vacancies. At frequencies before relaxation, the high polarization values are due to small polaron mechanisms associated with the presence of Fe2+. The relaxation processes are very much conditioned by the grain and domain sizes, the degree of deformation of the lattice and the crystallites, as well as the potential barriers in the grain boundaries. Values of the activation energy corresponding to the different relaxation processes were determined from fitting of experimental data, identifying thus the involved mechanisms, and an excellent agreement with those obtained from the temperature response [Raymond et al., J. Appl. Phys. 97, 084107 (2005)] was found. The relaxation processes studied here are an evidence of domain structure. © 2006 American Institute of Physics.