Microstructure and Phase Transition of MnO2-Doped Bismuth Layered-Structure Ferroelectrics

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The effects of MnO2 doping on the microstructure and phase transition of ferroelectric CaBi4Ti4O15 (CBT) ceramics with $x$ mol % of MnO2 (CBT–$x$M; $x = 0, 1, 3$) 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 Mn2+ 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 $T_{\text{C}}$ is independent of MnO2 doping within the experimental accuracy, and the dielectric constant satisfies the Curie–Weiss law above $T_{\text{C}}$. The temperature dependence of the soft mode with $x = 0$ and 1 shows a significant softening towards $T_{1}\sim T_{\text{C}} + 113$ °C (at $T_{1}$, the square of the soft mode frequency $\omega_{\text{s}}{}^{2}\rightarrow 0$). $\omega_{\text{s}}{}^{2}$ is approximately proportional to $T - T_{1}$. These findings indicate that the displacive nature of the phase transition is not affected by MnO2 doping, at least below $x = 3$.
2009-09-25