The origin of multiple dielectric relaxation processes in Fe-substituted CaCu3Ti4O12 ceramics

A new Fe3+ substituted CaCu3Ti4−xFexO12 (CCTFO, 0 ≤ x ≤ 0.2) ceramics prepared by solid-state reaction method has been systematically studied. Three new relaxation processes were discovered by means of dielectric properties spectra measurement. The first and second relaxation processes (I and II) were observed at room temperature for CCTFO (0 ≤ x ≤ 0.04), and the third relaxation process (III) appeared starting about 75 °C only for CCTFO (x = 0.01). These rare multiple dielectric relaxation processes urge us exploring the fascinating origin of giant dielectric constant in CaCu3Ti4−xFexO12 ceramics. Impedance measurement results showed that the resistivity of grains and grain boundaries increased while dielectric constant decreased dramatically in the frequency range of 1 kHz to 1 MHz with content x increasing. Based on the IBLC model, we can explain that relaxation process I origins from Maxwell–Wagner relaxation at grain boundaries. In the frequency range of occurring relaxation process II, dielectric constant decreased from 105 to 5 × 104 after surface polishing, which confirmed that the giant dielectric constant appearing at low frequency (<1 kHz) originated not only from the grain boundaries but also the surface layers in the new ceramics. For relaxation process III, permittivity plateau appears at lower frequency range starting from 100 Hz and high temperature (T ≥ 75 °C) may be relative to the concentration of localized charge carriers.