The effect of Sr doping on structural and dielectric properties of Ba₂Co₂Fe₁₂O₂₂ ceramics

Polycrystalline Ba_2−xSr_xCo₂Fe₁₂O₂₂ (0 ≤ x≤0.5) ceramics have been synthesized by the conventional solid-state-reaction method. From the analysis of X-ray diffraction and scanning electron microscopy, it can be obtained that the lattice parameters decrease monotonically with the increase in Sr doping content, while the average grain size presents a complicated trend. The dielectric measurements show that dielectric constants of all samples are larger than 10³ over a wide range of temperature and frequency (300–650 K and 1 kHz–1 MHz), which is identified as colossal dielectric constant. However, with the rise in Sr doping content, disparate evolutions of dielectric constant are found at 300 K and 600 K. A detailed study of dielectric response, impedance spectra and conductivity analysis suggest that the variation of dielectric constant at 300 K is dominated by the activation energy of relaxation, while that at 600 K is mainly in connection with the average grain size.