| The structural, ferroelectric and dielectric properties of A-site doped Sr2Bi4Ti5018 (SBTi), which belongs to the family of layer-structured ferroelectrics, have been studied. These studies will be helpful to understand the mechanism of the property variations of layer-structured ferroelectrics caused by A-site doping.A-sites in perovskite unit of SBTi are occupied by two kind of cations, trivalent Bi3+ and divalent Sr2+. The ferroelectric ceramics of SBTi doped by different lanthanide (La, Sm, Dy) and alkaline earth (Ca, Ba) have been prepared by solid-state reaction method. The structure was analyzed by X-ray diffraction. It is found that the structure of the samples has been changed little by A-site doping.The effect of substitution of some Bi3+ ions by trivalent lanthanide ions on properties of SBTi has been studied. Investigations of ferroelectric properties show that the remnant polarization 2Pr of SBTi ceramics is approximately 16.3 μC/cm2 under a driven electric field of about 125 kV/cm. The 2Pr increases at first, then decreases with the increase of doping content. The 2Pr of La doped SBTi maximizes at 18.4 μC/cm2 with doping content of 0.05. The 2Pr for Sm and Dy doped SBTi reach a maximum value of 18.2 μC/cm2 and 20.1 μC/cm2, respectively, when doping content is 0.01. The effect of lanthanide doping on ferroelectric properties is related to several factors such as the defect concentration and the structural distortion. The steady lanthanide doping at the A-site in SBTi would favour to lower the concentration of oxygen vacancies. The elimination of oxygen vacancies is helpful to weaken the influence of domain pinning and result in the increase of the 2Pr. On the other hand, the decrease of the 2Pr is attributed to less structural distortion caused by lanthanide doping. In addition, the incorporation of lanthanide ions into Bi2O2 layer may be another possible reason for the deterioration of the ferroelectric properties with higher doping content. La, Sm and Dy can decrease the coercive field Ec of SBTi. The Tc of the samples shifts to lower temperatures monotonously with lanthanide doping, which implies the less structuraldistortion. The strong hybridization between Bi 65 and O 2p orbit leads to a drastic distortion of the pseudo-perovskite blocks. However, when lanthanide ion without 6s electrons occupys the v4-site, the structural distortion and the Tc tend to decrease. Among doping ions, La doping leads to the Tc decrease more drastically than Sm and Dy doping. The hybridization between 4/electrons of Sm3+, Dy3+ and O2" 2p electrons might occur to a certain extent. But in the case of La-doped SBTi, such hybridization does not exist due to the absence of 4/ electron, which considerably favours to the relief of the structural distortion.The Tc of the samples increases with substitution of Ca2+ for Sr2+. The 2Pr increases at first, then decreases with the increase of doping content. The 2Pr of Ca doped SBTi maximizes at 21.1 /iC/cm2 with Ca content of 0.15. Substitution of Ba2+ for Sr2+ in SBTi degrades the ferroelectric properties monotonously. When all of the Sr2+ ions are substituted by Ca2+ or Ba2+ ions completely, the ferroelectric properties almost disappears in samples. Ca doping brings about the more pronounced structural distortion, which causes the higher Tc. The increase of 2Pr is also ascribed to the pronounced structural distortion. However, the 2Pr decreases with higher Ca content, which may be related to the following factors: the decrease of the whole shift of perovskite blocks along the a axis with respect to Bi2C>2 layers and the increase of defect concentration. Ba doping decreases the structural distortion, leading to smaller spontaneous polarization and degradation of the ferroelectric properties. The permittivity peaks of Ba2Bi4Ti5O|g become broad obviously, indicating a gradual ferro-paraelectric transition. The temperature at which the permittivity passes through a maximum depends on frequency, exhibits relaxer-type ferroelectric behaviors. |
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