High-temperature colossal dielectric response in RFeO3 (R=La, Pr and Sm) ceramics

Polycrystalline RFeO3 (R=La, Pr, and Sm) ceramics have been synthesized by a modified Pechini method. All the refined X-ray diffraction patterns reveal that the materials crystallized in orthorhombic structure with space group Pbnm. Clear grain and grain boundary are evident in PrFeO3 and SmFeO3 ceramics after sintering at 1200 °C for 4 h by scanning electron microscopy. Mixed valence of Fe2+/Fe3+ has been determined using X-ray photoelectron spectroscopy. High-temperature dielectric behavior and impedance spectrum over the frequency range of 100 Hz to 10 MHz are investigated at different temperatures from 300 to 600 K. These materials exhibit relaxation-like dielectric behavior and colossal dielectric constant in a wide temperature and frequency range. Impedance spectrum analysis indicates the ceramics to be electrically heterogeneous consisting of semiconducting grains and insulating grain boundaries (corresponding to high and low frequency electrical response, respectively), which played important roles in the high-temperature dielectric properties of RFeO3 ceramics.