| The effect of doping in A site on the ferroelectricity and relaxor-like dielectricity of bismuth layer-structured ferroelectrics (BLSFs) has been studied .The remanant polarization (2Pr) of SrBi4Ti4O15 (SBTi144) thin films is lower and the fatigue-endurance property deteriorates with the increase of switching pulse width. Recently, it was reported that substituting divalent-ions for Sr2+ in SBTi144 has enhanced the properties of SBTil44. However, there is no report on doping in A site by trivalent ions of SBTi144. The father investigationon SrBi4-xMxTi4O15 (M: trivalent ions) will be helpful to elucidate the mechanism of the property variationof the BLSFs upon doping.The ferroelectric thin films of SBTi144 doped by neodymium ions (SrBi4-xNdxTi4O15) have been prepared by Sol-Gel method. Some fractors in the process for preparing thin films, such as the precursor solutions, the pH value of the precursor solutions, the calcining and annealing temperature have dramatically affected the surface morphology and the ferroelectric properties of SBTi144 thin films. These results will be helpful to us to prepare other thin films.The ferroelectricity of SrBi4-xNdxTi4O15 (SBNT) thin films have been measured. The remanent polarization (2Pr) decreased with the increase of Nd content. Under an applied field of 375 kV/cm, the 2Pr of SBNT (x = 0.5) is about 6 Ccm-2, decreased by near 70% compared with that of SBTi144, which shows a higher value of 20 C-cm-2. The x-ray diffraction (XRD) pattern and atomic force microscopy (AFM) of SBNT thin films show that the more Nd content, the worst crystallization of the thin, films. The decrease of the polarization may relate to the following factors. Firstly, substituting smaller radii ions of Nd3+ for Bi3+ in SBTi 144 should bring about less structural distortion. Secondly, some Nd3+ ions may incorporate into Bi2O2 layers, which may destroy the origin effects of the insulated layers and the space chargecompensation. The fatigue characteristics of SBNT (x = 0.25) was measured with 50kHz switching frequency. The normalized nonvolatile polarization decreased 60% after 7.2 109 cycles.The relaxor-like dielectric properties induced by doping in A site of BLSFs have seldom been reported. In this paper, the dependence of dielectric constant on temperature with different frequency for Bi4 _ vLaxTi3O12- SrBi4-xLaxLi4O15, Sr2Bi4-xLaxTi5O18 and (Bi,La)4Ti3O12-Sr(Bi.La)4Ti4O15 (SrBi8-xLaxTi7O27) ceramics was investigated. The rare earth, La, concentration determines the ferroelectric characteristics of the compounds: for small x, they behave as typical ferroelectrics, but all of them tend to become relaxors when x grows.The Raman spectra of lanthanum doped SrBi4Ti4O15, Sr2Bi4Ti5O18 and Bi4Ti3O12-SrBi4Ti4O15 show that when La content is higher, some of La ions begin to incorporate into the Bi2O2 layers, similarity as suggested for La doped Bi4Ti3O12. The relaxation characteristics observed in SrBi4-xLaxTi4O15 (x =1.0), Sr2Bi4_xLa.xTi5O18 (x 0.5), SrBi8-xLaxTi7O27 (x = 1.50) are consistent with the softening of the mode lower than 30 cm-1, which indicate that La-doping may bring about the structured phase transition.The incorporation of La ions into Bi2O2 layers may destroy the original effects of the insulated layers and the space charge compensation, which should weaken the role of Bi2O2 layers as refraining the concentration of oxygen vacancies. Therefore, Defects can easily migrate and concentrate at the domain walls, which increase the local strain field and induce the microdomains. The apperance of ferroelectric and relaxor behavior may result from competition of the interaction of dipoles and random electric and strain fields. Then the ferroelectric relaxor behavior of La doped bismuth layer-structure ferroelectrics may be attributed to the random strain field induced microdomain state. |
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