The Bi <sub> 2 </ Sub> Of Wo <sub> 6 </ Sub> 15 </ Sub> Structure And Performance Of Symbiosis Of Bi <sub> 5 </ Sub> Tinbwo <sub>

The main interest of this thesis lies in the crystal structure, ferroelectric and dielectric properties of typical BLSFs Bi₂WO₆ (BW) and its intergrowth ceramics Bi₂WO₆-Bi3TiNbO9 (BW-BTN).Among the BLSFs, Bi₂WO₆ is the simplest member. The Bi3+ ions at A sites are shared with the Bi₂O2 layers. The Bi₂WO₆ ferroelectric ceramics with excess Bi₂O3 of 0.0, 2.0, 3.5 and 5.0 wt.% of the stoichiometric composition were prepared by the conventional solid-state reaction method. Their microstructure, ferroelectric properties, the concentration and mobility of the defects have been analyzed systemically. With increasing Bi content, the remnant polarization was decreased, and the broken-down voltage was increased. When the Bi excess increase to 5 wt.%, the broken-down field reach 600kV/cm. The optimum Bi excess, 3.5, lowered the oxygen vacancy concentration, while further Bi-addition brought about more defects. The activation energies fitted from cole-cole plots were 0.97 eV, 1.07 eV, 1.18 eV, and 1.33 eV, respectively. This suggests that the mobility of the defects was weakened by Bi-addition, which may be due to the increase of the ratio of the number of Bi₂O2 layers to that of the perovskite blocks and the insulating effect enhances.As for BW-BTN intergrowth ceramic, the X-ray diffraction (XRD) and Raman patterns show that the intergrowth structure has been formed and no second phases exist. The ferroelectric results show that the ferroelectric property of this material is very poor, and the hysteresis loop appears distorted. We have tried several methods including the changes in experimental approach, adjustment of raw powders and doping to improve its ferroelectric property. But no perfect effect has been received.In order to understand this intergrowth material further, the phase transition, dielectric relaxation and conductivity behaviors have been investigated with a low-frequency impedance analyzer (HP4192A). The temperature dependence of dielectric constant and loss (5 kHz≤f≤500 kHz) show that the phase transition appears around 600°C has obvious diffuse character. A modified Curie-Weiss law was used to study the diffuseness of this phase transition. The value ofγwas close to 2 which means a high extent of diffuseness. The poorly ferroelectric property of this intergrowth maybe have some relevant with this. A dielectric loss peak with relaxation-type characteristic is observed in BW-BTN ceramics in the temperature range of 200?400°C. This peak was proved to be associated with the oxygen vacancies through oxygen treatment and Nd-doping. The modified Cole-Cole relation was used to describe this temperature-dependent behavior of dielectric relaxation process. Its relaxation activation energy was determined to be 0.76 eV. The obtained broaden factorαwas around 0.4, which indicates a strong correlation among those relaxation units. With the reduction of oxygen vacancies through oxygen treatment and Nd-doping, the correlation among these units decreases and the activation energy increases.In addition, two distinct relaxation mechanisms were detected both in the plots of dielectric loss (tanδ) and the imaginary part (Ζ'') versus frequency in the frequency range of 10 Hz– 13 MHz. We attributed the higher frequency relaxation process to the hopping process of the oxygen vacancies inside the grains, while the other seems to be associated with the space charges bound at the grain boundary layers through the analysis and comparison of the value of relaxation energy and time. The ac impedance spectroscopy indicated that the conductivities at 625 K for bulk and grain boundary are about 1.12×10-2 S/m and 1.43×10-3 S/m, respectively.