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

Abstract

The effects of MnO 2 doping on the microstructure and phase transition of ferroelectric CaBi 4 Ti 4 O 15 (CBT) ceramics with x mol % of MnO 2 (CBT-x M; x ¼ 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 ¼ 3. The dielectric permittivity and dielectric loss show that the Mn 2þ and Mn 3þ ions are preferentially incorporated into the A-and B-sites, respectively, up to a limiting extent of approximately x ¼ 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 MnO 2 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 ¼ 0 and 1 shows a significant softening towards T 1 $ T C þ 113 C (at T 1 , the square of the soft mode frequency ! s 2 ! 0). ! s 2 is approximately proportional to T À T 1 . These findings indicate that the displacive nature of the phase transition is not affected by MnO 2 doping, at least below x ¼ 3.

Doping effects on dynamical physical properties of Ca0.6Sr0.4Bi4Ti4O15 ferroelectric ceramics with layered-perovskite structure

“…the internal modes of TiO 6 octahedron located >200 cm −1 and the lattice modes involved in the motion of the cations located <200 cm −1 . The detailed of the mode identification was reported in literature [10,26,27]. The line shapes of spectra are almost invariant at room temperature, while the scattering intensity decreases drastically due to the CeO 2 and MnO 2 doping.…”

“…At the low temperature region, the conductivity is presumably predominated by the extrinsic defects, while at the high temperature region ∼300-∼700°C conduction is predominated by the intrinsic defects [20]. It is known that in a single phase materials with homogeneous microstructure the conductivity depends on both the mobility and concentration of the charge carries and the conductivity can be presented by the following empirical equations [10,17]:…”

“…Figure 7 shows the temperature dependence of square of the soft mode peak frequency (w 2 s ), and it monotonically decreases towards the T 1 , the extrapolated temperature at which w 2 s goes to zero. The fact T 1 >T C is a common feature of a first order phase transition [10,25]. While the damping of soft mode randomly increase with the increase of temperature that can be explained by considering the local strain in the pseudo-perovskite unit.…”

“…The crystal structure of BLSFs consists of pseudo-perovskite layers interleaved with the bismuth oxide (Bi 2 O 2 ) 2+ layers along the c-axis [5][6][7][8][9]. The CaBi 4 Ti 4 O 15 (CBT) is one of the important member of BLSFs family for high temperature piezoelectric applications owing to its low electric conductivity, low dielectric loss, high T C (790°C), and easy to fabrication by the conventional solid state reaction [10][11][12][13]. However, the relatively low piezoelectric response and large temperature dependence is the main obstacle for the commercialization [14].…”