Structures, Ferroelectric And Dielectric Properties Of Bismuth Layer-structured Compounds

Bismuth layer-structured compounds are a family of ferroelectrics usually possessing high Curie temperature. These compounds play an important role in the integrated ferroelectrics industry. In this thesis, the lanthanide substitution was carried out in some typical bismuth layer-structured compounds and the variation of structure, the ferroelectric and dielelctric properties were investigated. Then, the relation between the structure stability and the average radius of A-site cations was discussed. The diffuse phase transitions of some Ba-based bismuth layer-structured compounds were also described and analyzed.With La³⁺ substitution, Bi_4-xLa_xTi₃O₁₂ underwent an evolution from orthorhombic to tetragonal structure. Accordingly, ferroelectricity in Bi_4-xLa_xTi₃O₁₂ was impaired and finally disappeared. Bi_4-xLa_xTi₃O₁₂ exhibited relaxor behaviors at x=1.2. When x exceeded 1.2, Bi_4-xLa_xTi₃O₁₂ was paraelectric at room temperature. Dielectric ceramics with dielectric constants higher than 125 and low tanδ values in the order of 10^-4 were attained when x>1.2. The author suggested that La³⁺ substitution have little effect on the long-range Coulomb forces but enhance the short-range repelling forces in favor of the stability of paraelectric structure. This may be the reason for the impairment of ferroelectricity in Bi_4-xLa_xTi₃O₁₂.Similar results were obtained in La³⁺ substituted SrBi₄Ti₄O₁₅. Solid solutions formed within the range of 04-xLa_xTi₄O₁₅. Curie temperature was significantly lowered with La³⁺ substitution. The phase transition became diffused and relaxor behaviors were found for x>0.8. The diffusion degree of the phase transition was enhanced with increasing x. The structure refinements of SrBi_4-x La_xTi₄O₁₅ at x=1.0 revealed that the external and internal oxygen octahedra were different both in the tilting around the crystallographic axis and in the deviation of Ti⁴⁺ from the equilibrium position. Further more, there was a cation disorder at the perovskite A-site and the cation site in (Bi₂O₂)²⁺ layer. This was believed to be responsible for the diffusion of the phase transition.To further clear the relationship between the factors contributing to theferroelectricity in bismuth layer-structured compounds, small Nd3+ was used to replace Bi3+ in Cal^TuOis. With increasing Nd3+ content, the orthorhombicity of the structure decreased, and the Curie temperature was lowered together with the suppression of the dielectric constant peak. This strongly implied that Bi3t should be crucial for the strong ferroelectricity in CaBi4Ti4Oi5 and its analogues, while the size mismatch between A-site cations and the provskite cuboctahedral cavity seemed to play a secondary role in the contribution to the ferroelectricity.Ba2Bi4Ti5Oi8, Sr2Bi4Ti5Oig, Ca2Bi4Ti5Oi8 and Ca3Bi4Ti6O2i ceramics were successfully prepared, while the efforts to synthesize Ba3Bi4Ti6O2 and Sr3Bi4Ti6O2i resulted in a mixture of Ba2Bi4Ti5Oi8 and BaTiCb, Sr2Bi4Ti5Oi8 and SrTiC>3 respectively. The structure stability of bismuth layer-structured compounds was closely related with the average radius of A-site cations. The forming of bismuth layered structure was proved to be a process in which the oxygen octahedra crystallize between two adjacent (Bi2O2)2+ layers. The phase transition of Ba2Bi4TisOi8 wasdiffused and that of Sr2Bi4TisOi8 was a normal one. Ca2Bi4Ti5Oi8 andwere proved to be ferroelectric. No dielectric constant anomaly was found in therange from room temperature to 800°C for Ca2Bi4TisOi8 and Ca3Bi4Ti6C>2i ceramics.The diffused phase transitions of BaBi2Nb2C>9, BaBi4Ti4O]5 and BaBisTiyC^ were investigated. BaBi2Nb2C>9 exhibited typical relaxor behaviors, but BaBi4Ti4Oi5 and BaBi8Ti7O27 were more like normal ferroelectrics. This can be well explained using the random-field theory. The dielectric properties of annealed BaBi2Nb2C>9 ceramics were nearly the same as those of as sintered nones, and this suggested that the cation disorder in BaBi2Nb2O9 should be an intrinsic property. The study with TEM revealed that there were polar nano-domains in BaBi2Nb2C>9. In addition, the stacking of perovskite layers along c axis was not so strict in the mixed bismuth layer-structured...