Tunability Of Crystal Structure And Physical Properties In Bismuth Ferrite Based Multiferroic Ceramics Via Element Combinations

Multifunctional materials（multiferroics,magneto-dielectric,spintronics etc.）have attracted increasing attention due to their possible applications towards storage materials and intriguing fundamental physics.Specifically,in the transition metal oxides,strong interplay between lattice,charge,spin and/or orbital degrees of freedom provides a fantastic playground to tune their physical properties.The control of magnetism by electric field is an important aspect for the development of low power spintronics.Among the naturally existing oxides,the presence of both ferromagnetism and ferroelectricity is a rare phenomenon,due to the incompatibility between magnetism and ferroelectricity.Bismuth ferrite is one of the rare multiferroic compounds in which ferroelectricity and magnetism coexist at room temperature.Moreover,it has many merits,such as antiferromagnetic and ferroelectric transition temperatures（Tc～1100 and TN～643 K）well above room temperature,large spontaneous polarization（Ps～100m μC/cm²）and so on,making it one of the most studied materials.However,the valence change in Fe ions and oxygen vacancies induced by the volatilization of Bi ions at high temperature make the bulk BiFeO₃ with impurity and serious leakage current,and make it difficult for BiFeO₃ to obtain saturation hysteresis loop and high residual polarization.In addition,the unique spatial modulation of the spiral magnetic structure makes it exhibit room temperature antiferromagnetic,which is harmful to the magnetoelectric coupling effect,and finally limits its practical applications.Therefore,the optimization of preparation methods and other modification methods have emerged to improve the ferroelectric and ferromagnetic properties of BiFeO₃ multiferroic materials.In view of the above issues,the preparation and the effects of diamagnetic metal ions combination at Bi site and non-magnetic metal ion doping at Fe site on the crystal structure transition,dielectric,ferroelectric,optical and magnetic properties of BiFeO₃-based multiferroic ceramics were systematically studied.The main research work and results of this thesis are as follows:1.In the study,Sr and Pb ions combination co-doped BiFeO₃ multiferroic ceramics have been prepared by a rapid solid phase reaction.The experimental results reveal that the crystal structure of BiFeO₃ transforms from the rhombohedral（space group R3c）structure to the cubic（space group Pm-3m）structure with the increment of Sr/Pb co-doping concentration.Meanwhile,Raman spectra suggest that the intensities of Bi-O bond vibrations gradually decrease.Furthermore,it is found that the dielectric constant and dielectric loss tangent of the samples measured in the frequency range of 100-107 Hz increase and decrease drastically at room temperature,respectively.Fortunately,the leakage current for BiFeO₃ considerably reduces while maintaining the ferroelectric property with Sr/Pb co-doping content increasing.The enhanced ferromagnetic properties of Sr/Pb co-doped BiFeO₃ multiferroic ceramics are mainly due to the synergistic effect of Fe²⁺-O-Fe³⁺ double exchange interaction and the effective suppression of antiferromagnetic cycloidal spin structure caused by the crystalline structure transition.2.In order to further improve the ferroelectricity and leakage current of BiFeO₃,Ca²⁺ ions with a smaller ionic radius than that of Bi³⁺ ions were selected to replace the above Sr²⁺ ion to obtain Ca/Pb ions combination co-doped BiFeO₃ ceramic.In order to compensate the valence balance caused by the Ca²⁺ dopant,the Pb ions present tetravalent state under the existence of Ca ions.It is found that with the increase of Ca/Pb concentration,BiFeO₃ also undergoes a crystal structure transition from rhombohedral phase（space group R3c）to cubic phase（space group Pm-3m）.Compared with Sr and Pb combination co-doping,Ca and Pb ions combination co-doping significantly improves the dielectric and ferroelectric properties of BiFeO₃ ceramics,and even reduces the leakage current by two orders of magnitude.The only downside is that the enhanced magnetization in Ca/Pb co-doped BiFeO₃ ceramics is not better than that of Sr/Pb co-doped BiFeO₃ ceramics.Based on the benefits of Ca doping,we can infer that Ca and Pb ions combination co-doping may induce some ferroelectric domains contributing to the polarization,but the damage to the magnetic structure of BiFeO₃ from the Ca/Pb co-doping is less than that from Sr/Pb co-doping.3.The K and Pb ions combiantion co-doped BiFeO₃ nanoparticles were prepared by a mild sol-gel method.As the co-doping content of K and Pb ions combination increases,accompanying Fe deficiency are introduced,the crystal structure transition of BiFeO₃ from rhombohedral R3c to cubic Pm-3m and the formation of the local distortion were achieved.With the increase of K and Pb ions combination contents,the band gap of BiFeO₃ nanoparticles decreases gradually,which is caused by the broadens of the conduction band and valence band,resulting from the crystal structure transition（R3c->Pm-3m）and local distortion induced by Fe deficiency.Furthermore,the saturation magnetization first decrease and then increase with the increase of the K and Pb ions combination co-doping.The former decrease is due to the reduction of the ferromagnetic coupling within the pseudo-cubic（111）planes induced by Fe deficiency is larger than the increment of the ferromagnetic interaction under the low content of cubic phase for the samples with 0.02 ≤ x ≤ 0.05.The latter increase is mainly due to the potential ferromagnetic interactions released from the collapse of spiral spin cycloid structure within the larger content of cubic phase for x = 0.1 sample are much larger than the decrease of the ferromagnetic coupling caused by the Fe deficency.4.In this work,the optical bandgap and magnetization of Bi₂Fe₄O₉ and Bi₂Fe_3.6Me_0.4O₉（Me = Al and Ti）ceramics have been investigated by UV-Vis diffused reflectance spectroscopy and magnetization hysteresis（M-H）curves,respectively.It is found that the optical bandgap varies from 2.06 eV for Bi₂Fe₄O₉ down to 2.01 eV for Bi₂Fe_3.6Al_0.4O₉ and 1.86 eV for Bi₂Fe_3.6Ti_0.4O₉,respectively.Simultaneously,accompaning an apparent increase of low temperature magnetization for Bi₂Fe_3.6Ti_0.4O₉ ceramics,while the magnetization for Bi₂Fe_3.6Al_0.4O₉ nearly unchanged are also observed.For Bi₂Fe_3.6Me_0.4O₉（Me = Al and Ti）ceramics,the variation in bandgap and magnetization can be ascribed to the site-specific substitution of Fe³⁺ in FeO₄ tetrahedra or FeO₆ octahedra from the dopant ions.Our work offers a site-specific substitution for manipulating the bandgap and magnetization in Bi₂Fe₄O₉ ceramics,thus boosting potential applications in future photovoltaic devices and photocatalytic materials.