Structure, Dielectric And Ferroelectric Properties Of Some Fe-Based Complex Perovskite Ceramics

As an important group of dielectric materials, Fe-based complex perovskites ceramics have drawn losts increasing scientific attention, while the physical nature of their dielectric behavior has always been very controversial. In the present thesis, the structure, magnetic, dielectric and transport properties of the Fe-based complex perovskite ceramics were investigated systematically. Meanwhile, the conversion between the diffuse ferroelectrics and relaxor of their solid solutions was demonstrated. The relationship between the physical properties and the structure has been discussed, and the physical origin of dielectric relaxation and the relaxor ferroelectric behavior was revealed.XRD data suggested that Ca(Fe1/2Ta1/2)O3ceramic had orthorhombic structure with B-site disorder. There was only one dielectric relaxation observed in the low temperature ranges, respectively. Compared with Ba(Fe1/2Ta1/2)O3, Sr(Fe1/2Ta1/2)O3and Ba(Fe1/2Nb1/2)O3, the much lower dielectric constant and lower dielectric loss were determined in Ca(Fe1/2Ta1/2)O3because of the obviously weaker Fe2+/Fe3+mixed-valent structure. The oxygen annealing had little influence on oxygen vacancies and Fe2+/Fe3+mixed-valent structure.The X-ray powder diffraction analysis confirmed that Sr(Fe1/2Ta1/2)O3has a B-site disordered orthorhombic structure in space group Pbnm(62). Only one broadened dielectric peak with strong frequency dispersion was observed in present ceramic, which was significantly different from that for the analogue Ba(Fe1/2Nb1/2)O3and Ba(Fe1/2Ta1/2)O3. The strong dependence of sample thickness and electrode material indicated that the dielectric relaxation behavior at lower frequency was due to the interface effects. The present ceramic was spin glass state with slight ferromagnetic behavior below the Neel temperature (20K). The co-presence of Fe2+and Fe3+was comfirmed by the μff value.XRD analysis confirmed that the structures of Pb1-xBax(Fe1/2Nb1/2)O3solid solutions were cubic, and the dielectric nature changed from diffuse ferroelectric to relaxor ferroelectric with increasing x, while the phase transition temperature Tc (or Tm) decreased monotonously. The diffuse ferroelectric phase transition was observed in the ceramics with0≤x≤0.05. For Pb1-xBax(Fe1/2Nb1/2)O3with0.1≤x≤0.2, relaxor ferroelectric behavior was determined, and Vogel-Fulcher equation and new glass model could be used to describe the relaxor behavior.The crystal structures of all Pb1-xBax(Fe1/2Nb1/2)O3compositions were cubic and the cell volume indicated a sudden change at x=0.075. Pb1-xBax(Fe1/2Nb1/2)O3ceramics with x>0.075were paraelectric, while those for x<0.075were ferroelectric at room temperature. The phonon modes revealed that the differences of local structures of the ceramics might mostly caused by the disappearing of off-center BO6octahedron. The ferroelectric related distortion still could be discovered in paraelectric solid solutions with x>0.075. The modes related to Fe-O-Fe were stable with increasing Ba-content, while the phonon mode corresponded to Nb-O-Nb changed intensively. Meanwhile, the other three Nb-O related modes:the B-O asymmetric stretching mode near700cm-1, the Alg mode of the rigid B’-O-B"(Fe-O...Nb) bonds near780cm-1, the B-O bond related mode near850cm-1also presented the V-types inflection near x=0.075. These behaviors revealed that the ferroelectricity in the present system was due to the displacement of Nb cation. Moreover, the Nb-rich areas should exhibit the stronger ferroelectricity than other areas.According to XRD analysis, the structure of Ba1-xBix(Fe(1+x)/2Nb(1-x)/2)O3(x=0.1,0.3,0.5,0.7,0.9) solid solution ceramics varied from original cubic symmetry to rhombohedral distorted perovskite. With increasing x, the two dielectric abnormities caused by Fe2+/Fe3+mixed-valent structure tapered off, the dielectric loss decreased evidently. Oxygen annealed Ba0.1Bi0.9(Fe.9sNb0.05)O3ceramics prepared by SPS showed dielectric abnormity at613.1K. This temperature was the antiferromagnetic transition temperature confirmed by DSC. The phenomenon indicated the strong magnetodielectric effect at Neel temperature. Even at room temperature the magnetodielectric coefficient was as high as-0.4%.