The Effect Of A-doping And Ion Occupancy On The Multiferroic Properties Of Layered Perovskites

The layer-structured bismuth Aurivillius phase mulitferrocis Bi₄Bi_n-3Fe_n-3Ti₃O_3n+3 consist of（Bi₂O₂）²⁺ and perovskites-like[A_m-1B_mO_3m+1]^2-,which periodicity stacked along c direction.It has attracted much attention for the coexistence of ferromagnetism and ferroelectricity above room temperature.In the layer-structured bismuth oxides,the non-centrosymmetric structure and the distribution of Fe/Ti ions in the octahedral B site accompany with strong interaction between adjacent ions showing a fascinating ferroelectric and ferromagnetic properties.It provides a possibility to obtain multiferroics at room temperature and has great potential application in high-density storage,sensors,and spintronic devices.In this thesis,based on four-layered structure Bi₅Fe_0.5Co_0.5Ti₃O₁₅（BFCT）materials,the effects of A-site doping with different lanthanides,concentration of oxygen vacancy,and synthetize processes on microstructure,ferromagnetic,ferroelectric were investigated.The powders were synthesized using the solid-state reaction method,and the corresponding ceramics were sintered at high temperature.The influences of A-site doping BFCT ceramics with different lanthanides on the microstructure and multiferroic properties were firstly investigated.The concentration of doping lanthanum effects on the material microstructure and ferromagnetic,ferroelectric,dielectric was also discussed.Finally,the effect A-site doping with differences processes on microstructure,ferromagnetic,ferroelectric and dielectric of BFCT ceramics was compared.Chapter 1 is the background of multiferroic materials,including the characterization,development and progress of it.Chapter 2 introduces the methods of preparing BFCT ceramics and the fundamental of measurement for microstructure and properties of the corresponding materials.In chapter 3,it introduced the investigation of microstructure,ferroelectric and ferromagnetic properties of La doping BFCT with inserting process.With A-site La-doping amounts of x= 0,0.25,0.50,0.75,1.0,the Bi_1-xLa_xFe_0.5Co_0.5O₃ were inserted into Bi₄Ti₃O₁₂.It is found that inserting the magnetic unit into the ferroelectric host with La-doping can influence the multiferroic properties of samples,and the microstructure changed with the increasing of doping amounts.Besides,further comparing fund that both the magnetization and polarization reached the maximum at x=0.75.In chapter 4,the layer-structured bismuth perovskite Bi_4.25R_0.75Fe_0.5Co_0.5Ti₃O₁₅（R=La,Pr,Sm,Eu,Gd,Dy）ceramics were prepared by conventional solid state reaction method,with the purpose to compare the effects of A-site doping with different lanthanides on microstructure,ferromagnetism and ferroeletricity of multiferroics.It is revealed that the structure and properties parameters of samples were changed regularly with the increasing of ratio of doping elements.Furthermore,it is considered that the concentration of oxygen vacancy in samples may influence the properties during the process,and the La-doping sample was sintered at different atmospheres.It is found that the concentration of oxygen vacancy in samples has a remarkable effect on their multiferroic properties,especially on the magnetism and phase transfer temperature.In chapter 5,considering that the doping ions taking various positions in perovskite structures may have different influence on properties of materials,we prepared three samples with three different solid state reaction processes which could regulate the doping position.It inserted Bi_0.25La_0.75Fe_0.5Co_0.5O₃ and BiFe_0.5Co_0.5O₃ into Bi₄Ti₃O₁₂ and Bi_3.25a_0.75Ti)3O₁₂,respectively.In addition,the third sample was prepared by conventional solid-state reaction.After comparing the microstructure of samples,it is observed that all the samples with three different preparing processes are four-layered perovskite with an observed ferromagnetic and ferroelectric behavior.The transfer temperatures are obvious divided in samples with three different preparing processes.Chapter 6 is the summary of the whole thesis and forecasting the prospect of research direction.