| This thesis is aimed at the effect of lanthanoidal A-site modification on the multiferroic properties of Bi5Fe0.5Co0.5Ti3O15(BFCT) ceramics. The microstructure, ferroelectric, magnetic and dielectric properties of BFCT ceramics are characterized and the underlying mechanisms are also discussed.The influence of different excessive Bi2O3 content on the properties of BFCT ceramics are studied in the first part of this thesis. The XRD patterns for BFCT ceramics having excessive Bi2O3 molar ratio of-1%、0、2.5% and 5% confirmed the pure four-layer Aurivillius phases in all the samples. It is seen that the ferroelectric and magnetic behavior is undesirable for BFCT samples with the stoichiometric and deficient -1% Bi2O3 content. Due to the volatilization of Bi in high temperature sintering, the resulting oxygen vacancies appeared in the sample. For BFCT having bismuth excess of -1% and 0, the amount of oxygen vacancies is greater than the rest two samples. When an appropriate amount of bismuth excess is added into the samples, the multiferroic properties are therefore enhanced due to the compensation of existing oxygen vacancies. Our experiment shows that the 5% excess amount of bismuth for BFCT samples is an optimal value.On the basis of appropriate Bi2O3 excess, we do some research on the A site doped with Nd ions. Bi4.15Nd0.85Fe0.5Co0.5Ti3O15 (BNFCT) ceramics are prepared using the traditional solid state reaction method. X-ray diffraction patterns show that the Aurivillius phase is formed without any impurities. Raman scattering is used to indicate the distribution of the Nd ions and the vibration of the phonon modes corresponding to A-site ions and B-site octahedra were discussed. The ferroelectric hysteresis loops show the remanent polarization (2Pr) of BNFCT is 16 P C/cm2, which is twice as high as that of non-doped sample. It is considered that the suppression of oxygen vacancies and the increased lattice distortion due to Nd modification contribute to the enhancement in ferroelectricity. The remanent magnetization (2Mr) is also apparently increased up to 0.66 emu/g. X-ray photoelectron spectra show that the amount of Fe2+ ions decreases while that of Co3+ ions increases, which contributes to the more probability of spin-canting of the Fe-based and Co-based sublattices via Dzyaloshinskii-Moriya (DM) interaction.In the end, the A site doping of BFCT materials with different ionic radius is studied. Bi4.15A0.85Fe0.5Co0.5Ti3O15 (BAFCT) (A=Gd, Dy, Sm, Nd) ceramics are prepared by solid state method. X-ray diffraction patterns show that all samples have formed a four-layered Aurivillius phase. Due to the four doping ions with different ionic radius, the doped samples produced different degrees of lattice distortion, which was in favor of the generation of spontaneous polarization, as a result the ferroelectric properties of the all samples have increased. The 2Pr of the samples are 5.62 μC/cm2,4.61 μC/cm2,11.16 μC/cm2 and 15.7 μC/cm2, respectively. The A-site modification on BFCT ceramics also contribute to suppression of oxygen vacancies. Due to the influence of the lattice distortion of the samples, the magnetic property was enhanced by different degree, because the influence of lattice distortion due to substitution contribution positively to the angle of spin-canting and the coupling distance which promote the coupling of Fe3+-O-Co3+. The 2Mr of the samples are 0.78 emu/g,0.41 emu/g,0.66 emu/g and 0.56 emu/g, respectively. The magnetic changes with temperature is also tested, we found that the phase transition temperature of the samples are about 173 ℃,82 ℃,114 ℃ and 106 ℃, respectively. |
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