Giant Dielectric Response Of A (Fe_0.5B_0.5)O₃ (A=Ba, Sr; B=Nb, Ta) Ceramics And Its Modification

In the present thesis, the microstructure, dielectric and magnetic characteristics in complex perovskite A(Fe0.5B0.5)03 (A=Ba, Sr; B=Nb, Ta) ceramics were systematically investigated, and the physical nature and structural origins for the dielectric relaxations and the giant dielectric response were revealed, and the modification of the present materials was investigated.Two dielectric relaxations following Arrhenius law were observed in Ba(Feo.5Nbo.5)03 ceramics at 150-400 K and 400-550 K, respectively, and there was a giant dielectric constant step (ε'～30,000) between them. The low-temperature dielectric relaxation was nearly a Debye relaxation with the intrinsic nature, attributed to the charge hopping between Fe2+ and Fe3+ ions. The high-temperatrue dielectric relaxation could be suppressed by O2-annealing which was suggested to the results of defects. Moreover, the giant dielectric constant step was resulted from the competing balance of the low and high temperature relaxations. Ba(Fe0.5Nb0.5)O3 was antiferromagnetic with a slight ferromagnetic moment below Neel temperature, and above this temperature the weak ferromagnetic nature was revealed.Similar to the situation in Ba(Fe0.5Nb0.5)O3, two dielectric relaxations at low-and high-temperatures following Arrhenius law were observed in Ba(Fe0.5Ta0.5)O3 cermics. The linear hystereses loop at low temperature indicated that Ba(Fe0.5Ta0.5)O3 cermics was not a relaxor ferroelectrics. Ba(Fe0.5Ta0.5)O3 was antiferromagnetic with a slight ferromagnetic moment below 20 K, and above this temperature the weak ferromagnetic nature was indicated. The 57Fe Mossbauer spectra at room temperature showed that the weak ferromagnetic moment at room temperature in Ba(Fe0.5Ta0.5)O3 ceramics did not origin from 57Fe.Ba[(Fe0.9Al0.1)0.5Ta0.5]O3 solid solution was obtained in the Al-substituted (10mol%) Ba(Fe0.5Ta0.5)O3 ceramics, and the dielectric and magnetic characteristics were evaluated. The extended giant dielectric constant step and the significantly reduced room-temperature dielectric loss were obtained in the present modified ceramics. The weakened ferromagnetic ordering was revealed compared with the Ba(Fe0.5Ta0.5)O3 end-member. The results of57Fe Mossbauer spectra at room temperature suggested that oxygen vacancies appeared to play an important role in the room temperature ferromagnetism in Ba(Fe0.5Ta0.5)O3 ceramics.Ba[(Fe0.5Nb0.5)1-xTix]O3 (x=0.2,0.4,0.6,0.8,0.85,0.9 and 0.95) solid solutions were synthesized by a standard solid-state reaction technique. The structure of Ba[(Fe0.5Nb0.5)1-xTix]O3 solid solution changed from cubic to tetragonal with increasing x. A dielectric relaxation following Arrhenius law similar to that in Ba(Fe0.5Nb0.5)O3 was observed at lower temperature in the composition range 0.2≤x≤0.8. A relaxor ferroelectric, diffused ferroelectric and classical ferroelectric was observed for x=0.85,0.9 and 0.95, respectively. The Ba[(Fe0.5Nb0.5)1-xTix]O3 (x=0.85,0.9 and 0.95) solid solutions exhibited typical ferroelectric P-E hysteresis behavior below the transition temperatures.The modified low-and high-temperature dielectric relaxations with extended giant dielectric constant step were observed in Sr[(Fe0.5Nb0.5)1-xTix]O3 solid solutions. The dielectric constant of Sr[(Fe0.5Nb0.5)1-xTix]O3 solid solutions decreased first and then incressed with increasing SrTiO3 content at 1 kHz, while the dielectric loss was suppressed monotonically. At x=0.8, the dielectric loss at room temperature and 1 kHz was 0.16 which was much smaller than that for x=0 (tanδ=0.37), while the dielectric constant was increased to 6815, and the temperature stability was also improved. This indicated that the dielectric charicateristics of Sr(Fe0.5Nb0.5)O3 ceramics could be significantly improved by forming solid solid solution with SrTiO3.