High temperature dielectric response in R3Fe5O12 (RÂ =Â Eu, Gd) ceramics

Polycrystalline R3Fe5O12 (R = Eu, Gd) ceramics are synthesized by a sol-gel method. The X-ray diffraction patterns show that both materials exhibit the same cubic structure with space group Ia3¯d. The dielectric behaviors and impedance spectra in the frequency range from 100 Hz to 10 MHz are investigated at different temperatures from 300 K to 700 K. The materials exhibit relatively large dielectric permittivity and low dielectric loss in a wide temperature and frequency range. Two dielectric relaxation processes are observed in the measurements. The high frequency relaxation is shown to originate from the electrons hopping between Fe2+ and Fe3+, while the low frequency relaxation most likely arises from the doubly ionized oxygen vacancies. The impedance spectra indicate that the ceramics are electrically heterogeneous consisting of semiconducting grains and insulating grain boundaries (corresponding to the high and low frequency electrical responses, respectively), and the heterogeneous structures play important roles in the dielectric properties.