Phase Transition, Thermal Expansion And Magnetoelectric Effect On BiFeO₃Based Multiferroic Ceramics

Multiferroic BiFeO3with magnetic and ferroelectric coupling property has become one of the hottest topics of physics and materials sciences on the current, which has great potential application in the memory storage device. In this work, the phase transition, thermal expansion and magnetoelectric effect on BiFeO3based multiferroic ceramics have been studied. The major interesting results can be summarized as follows.1. The phase transition during the reaction sintering process of BiFeO3ceramics with different molar ratio of Bi2O3/Fe2O3and heating rate in air was studied via high temperature x-ray diffraction technique. And the thermal stability of BiFeO3, Bi25FeO39and Bi2Fe4O9ceramics with varying the cooling rate was also studied by such technique. Results show that the phase transition from monoclinic to cubic for Bi2O3was well done, which usually taken place at700-800℃. The Fe2O3did not react with Bi2O3to form BiFeO3until that transition finished. When the reaction temperature rose up to800℃, the impurity phases such as Bi25FeO39and Bi2Fe4O9synthesised simultaneously, more Fe2O3surpluses. The content of impurity phases mainly depend on the molar ratio of Bi2O3/Fe2O3, and1.03:1is optimum. Meanwhile it is indirect proportion to the heating rate, which may relat to the procedure of phase nucleation, decomposition and the transition between these phases. In addition, these phases are not in thermodynamic stable state during the cooling process between900-600℃. The content of impurity phases may relate to the cooling rate due to the metastability of BiFeO3.2. a) Ba, Pr co-doped BiFeO3samples are synthesized by modified solid state reactions. The structural, magnetic, and ferroelectric properties of polycrystalline Bi1-2.5xPri.5XBao.5XFe03(x=0.05,0.1) ceramics are studied systematically. A structural phase transition of Rhombohedral-to-Cubic occurs in Bi0.75Pr0.15Ba0.1Fe03sample. The strong enhancement of the magnetization also confirmed that structural distortion and collapse of the spatial spin structure are the origin of spontaneous magnetization respectively. The co-doping of Ba and Pr in Bi1.2.5xPr1.5XBaxFe03samples, in which Ba2+and Pr4+molar proportion as1:1, can reduce the possible charge defects without the change of iron valence. Pr4+may compensate the charge, and decrease the magnetism of BFO, which confirms that charge defects are another origin of spontaneous magnetization. The band gap decreases sharply with Pr doping from5.72eV (x=0.05) to0.18eV (x=0.1), which destroy the ferroelectric property. b) The structural, magnetic, and ferroelectric properties of polycrystalline Bi0.8-xPrxBa0.2Fe03(x≤0.1) ceramics are studied systematically. The symmetry of the unit cell with Ba and Pr co-doping in BiFeO3in the doping range remains the cubic with the space group of Pm3m. The additive of Pr in (BiBa)FeO3reduces the unit cell volume together with varying the lattice parameters without changing the unit cell symmetry. The distortion of FeO6octahedron resulting from the additive of Pr may induce the local collapse of the spatial spin structure, which may enhance the spontaneous magnetization significantly. The remnant magnetization (Mr) increases monotonically with increasing the content of Pr, which probably originates from the ferromagnetic arrangement of Pr-ions and Fe-ions in the unit cell. The conductivity increases in a wide temperature regime result in an obvious leakage observed with increasing the Pr concentration. 3. The phase transition and thermal expansion of Bi1-2.5xPr1.5xBaxFe03(x=0.05,0.1) polycrystallines is studied by high temperature x-ray diffraction in the temperature range of25-800℃. A structural phase transition of Rhombohedral-to-Cubic occurs for Bio.875Pro.o75Bao.o5Fe03sample in the temperature of600-700℃, which may relate to its unstable rhombohedra distorted structure with the space group R3c. The rarely decomposition of these samples indicates that the Pr, Ba co-doped make the BiFeO3ceramics more stable. The thermal expansion determined by the temperature dependence of the unit-cell lattice parameters and volumes for Bi1-2.5xPr1.5xBaxFe03samples is also investigated, which shows an isotropic and positive behavior. The average thermal expansion coefficient decreases with increasing x. We argue that the cubic crystal structure with the high symmetrical of the space group Pm3m may be more stable for Bi0.75Pr0.15Bao.iFeO3sample, which may explain the reason why no phase transition occurs and its lower thermal expansion efficiencies. An obvious change in the slope of the linear fitted lines between300℃and400℃suggests a possible antiferromagnetic-paramagnetic transition, which occurs around the Neel temperature of the Bi1-2.5xPr1.5XBaxFe03samples.