Sol-Gel Synthesis And Characterization Of Single-phase Multiferroic BiFeO₃

Abstract Bismuth ferrite BiFeO₃ with perovskite structure is one such material and is of particular interest in terms of practical applications, because both its electrical and magnetic ordering temperatures are well above room temperature. The crystal symmetry of pure BiFeO₃ belongs to the space group R3c. The rhombohedral unit cell parameters are ar=5.634 (?) andα_r=59.348°. It has ferroelectric properties with high Curie temperature (TC⁸50℃) and antiferromagnetic behavior with Néel temperature (T_N³70℃) at room temperature.The main problems of study on BiFeO₃ can be concluded: the origin of high polarization, the relation of magnetoelectricity to structure, synthesis method is complicated and phase pure sample is difficult to obtain and so on. Sol-gel method has many advantages in synthesis of ferroelectrics, such as simple device, easier composition control, better homogeneity. It is a useful method and can be in synthesis of BiFeO₃.In this dissertation, BiFeO₃ powders and thin films have been successfully synthesized by sol-gel method. The obtained samples were characterized by X-ray diffraction(XRD),X-ray photoelectron Spectroscopy(XPS),Thermal analysis (TG-DTA),Scanning electron Microscopy(SEM),Raman spectroscopy and Scanning tunneling microscopy(STM).The major work in this dissertation includes the following three parts:1. Sol-gel Synthesis and structural property of BiFeO₃ powders.BiFeO₃ powders were synthesized by sol-gel method. We studied the influence of the synthesized conditions (such as temperature and the volume of 2NHNO₃). We have obtained pure phase BiFeO₃ by optimized the synthesis conditions. The X-ray diffraction showed the powders are of a rhombohedrally distorted perovskite structure, the crystal parameters are ar=5.6107 (?) andα_r=59.478°. The structure instability was investigated by variable temperature X-ray diffraction. It was found BiFeO₃ crystal lattice constants increased with temperature gradually, and the powder show a positive thermal expansion property, the thermal expansion coefficient isα=1.1327×10^-5/℃.2. The physical chemistry property of the BiFeO₃ powders.The XPS results show that the powders are pure phase, and there no Fe2+ in the powders. TG-DTA plots indicate a phase transition at 837℃. We also studied the Raman spectra of the powders, the peaks of Raman graph appeared at 168 cm^-1, 261 cm^-1, 477 cm^-1 and 520 cm^-1, respectively. The Raman lines of BiFeO₃ powders exhibit a few Conspicuous broadening peaks. Based on the XRD pattern, a simulation is carried out using rhombohedral lattice type with the R3c space group. The simulation result and the difference between experiment and simulation are also simultaneously shown. The Rietveld refinements of crystal structure parameters are listed. We found the atomic positions, lattice parameters, and interatomic distances show significant distortion.3. Growth and characterization of BiFeO₃ thin films on Si subtract via sol-gel methodWe fabricated BiFeO₃ films on Si substrate, optimized the synthesis conditions of the thin films. BiFeO₃ films were characterized using x-ray diffraction and SEM, AFM. The results show that the film is pure phase, and the film shows smoother surface and relatively dense structure with finer crystallites.