Microstructure and phase transition of MnO₂-doped bismuth layered-structure ferroelectrics

The effects of MnO₂ doping on the microstructure and phase transition of ferroelectric CaBi₄Ti₄O₁₅ (CBT) ceramics with x mol % of MnO₂ (CBT-xM; x = 0;1;3) have been studied. The single-phase crystal structure and the plate-like grain morphology are clearly observed for all compositions, while extended grains are observed with x = 3. The dielectric permittivity and dielectric loss show that the Mn ²⁺ and Mn³⁺ ions are preferentially incorporated into the A- and B-sites, respectively, up to a limiting extent of approximately x = 2. The ac conductivity decreases and the activation energy increases with increasing Mn content owing to the decrease in the number of conducting carriers and increase in covalence, respectively. The Curie temperature T_C is independent of MnO₂ doping within the experimental accuracy, and the dielectric constant satisfies the Curie-Weiss law above T_C. The temperature dependence of the soft mode with x = 0 and 1 shows a significant softening towards T₁ ∼ T_C + 113°C (at T ₁, the square of the soft mode frequency ω² _s → 0). ω²₂ is approximately proportional to T - T₁. These findings indicate that the displacive nature of the phase transition is not affected by MnO₂ doping, at least below x = 3.