Structure, Dielectric Characteristics And Ferroelectric Transition Of Ba-Based Filled Tungsten Bronze Ceramics

In the present thesis, the structures, dielectric and ferroelectric properties with the phase transition were systematically investigated for the Ba-based filled tungsten bronze niobate ceramics, the effects of tetragonal Al-site cations on the structure and characteristics were emphasized, and the structure origins of relaxor nature, the ferroelectric or ferrielectric order and the low temperature dielectric relaxations were revealed.In the Ba5RTi3Nb7O30 (R= La, Nd, Sm) ceramics, the tetragonal tungsten bronze structure is formed at room temperature. The space group was P4/mbm for R= La; while for R= Sm and Eu, the space group was P4bm. Ba5YTi3Nb7O30 could not form the stable tetragonal tungsten bronze structure at room temperature. With the increasing of △R between the radius of pentagonal sites (A2) and tetragonal (Al) cations, the relaxor nature turns weaker in Ba5RTi3Nb7O30 (R= La, Nd, Sm): Ba5LaTi3Nb7O30 is a typical relaxor:broad dielectric peaks with strong frequency dispersion. The dielectric peak temperature versus frequence curve is well obeyed with the Vogel-Fulcher relationship. With the increasing of △R, even both the Ba5NdTi3Nb703o and Ba5SmTi3Nb7O30 exhibited the relaxor nature, the frequency dispersion phenomenon turned weaker, and the dielectric peak temperature increased. For all compositions, dielectric anomalies were observed below the phase transition temperatures, which may be associated with the low temperature structural modulation, like the oxygen octehedra distortion in the ab plane.For Ba5SmTi3Nb7O30, when Ba2+ was substituted by the smaller Ca2+, frequency dispersion in dielectric peaks was replaced by the sharp peaks at the phase transition temperature in Ba4CaSmTi3Nb7O30, exhibiting the normal ferroelectric with first order phase transition. According to the crystal structural refinement in Ba4CaSmTi3Nb7O30, the bond distance of [Ti/Nb(2)-O(5)] in ab plane decreased, indicating that the distortion of oxygen octahedra turned weaker due to the smaller Ca2+ filled in tetragonal Al site, which helped to establish the long-range ferroelectric order. In Ba5SmTi3Nb7O30, two cations with different radius and charges filled in the tetragonal Al site, which might form the micro electrical fields or the micro strain. Compared with the micro electrical fields, the micro strain was a more important role in determining the relaxor nature for Ba5SmTi3Nb7O30.In the Ba5RTi3Nb7O30 (R=La, Nd, Sm) tungsten bronze ceramics, both the remanent polarization and maximum polarization decreased at low tempaertures. In some relaxors of perovskite structure, similar phenomenon had defined as the "re-entrant relaxor" behavior. This phenomenon may be ascribed to the complex cation configurations in tungsten bronze structure and the structural modulation, which may impede the establishment of long-range ferroelectric order at low temperatures. On the other hand, the local inhomogeneity and structural fluctuation increased the energy barrier for reversed ferroelectric domain nucleation and growth, which finally increased the coercive field.In order to investigate the effect of A cations on the "re-entrant" relaxor behavior, Ba5LaxSm1-xTi3Nb7O30 (x=0.1,0.25,0.5,0.8,0.9,0.95) tungsten bronze ceramics were synthesized, and the low temperature dielectric relaxation and ferroelectric were investigated. With the increasing the amount of La in Al sites, relaxor ferroelectric property became stronger, which might be due to the larger cations radius and the distortion of oxygen octahedra nearby. With more La in Al sites, the polarization dropped more rapidly at low temperatures. From the polarization versus temperature curves, we proved that the more ordered cation occupation and micro structure would help to establishment the long-rang ferroelectric order and weaken the "re-entrant" relaxor behavior. On the other hand, the order structure will weaken the pining effect for domain wall motion and reduce the energy barrier for domain switch at low temperatures.For Ba4R2Ti4Nb6O30 (R= La、Nd、Sm、Eu) tungsten bronze structure ceramics, the dielectric properties and phase transition had strong relationship with cation radius difference between A2 and Al cations, like that observed in Sr based tungsten bronze ferroelectric ceramics. Ba4La2Ti4Nb6O30 is a typical relaxor ferroelectric, with strong frequence dispersion in dielectric curves. For Ba4Nd2Ti4Nb6O30, Ba4Sm2Ti4Nb603o and Ba4Eu2Ti4Nb6O30, sharp dielectric peaks emerged suggesting the normal first order phase transition. The Curie-Weiss law was well fitted for all compositions. For Ba4Nd2Ti4Nb6O30, the To is around 100K; while for Ba4Sm2Ti4Nb6O30 and Ba4Eu2Ti4Nb6O30, the To is around -522.36K and -317.9K, respectively. The negative To in Ba4Sm2Ti4Nb6O30 and Ba4Eu2Ti4Nb603o indicated the different structural change from Ba4Nd2Ti4Nb6O30. From the hysteresis loops measurement, we observed the quite different polarization nature and phase transition type with different R cations in Al sites. At room temperature, Ba4La2Ti4Nb6O30 is paraelectric phase with linear hysteresis loops. When cooling to low temperatures, non-linear hysteresis loops began to appear with small remanent polarization Pr at 193K. At high temperature, Ba4Nd2Ti4Nb6O30 is paraelectric phase with no hysteresis loops. When cooling to 393K, remanent polarization suddenly appeared, this is in accordance with the characteristic of first order phase transition in normal ferroelectric. While for Ba4Sm2Ti4Nb6O30 and Ba4Eu2Ti4Nb6O30, double hysteresis loops had been observed at 473K, which suggested the antiferroelectric order at high temperatures. When cooling to low temperatures, remanent polarization began to appear, suggesting that ferroelectric phase was the more stable phase, and there was the tendency for transforming into ferrielectric phase when cooling down. At room temperature, ferroelectric domain had been observed in PFM images, proving the non-centrosymmetric micro structure in Ba4Sm2Ti4Nb6O30 and Ba4Eu2Ti4Nb6O30.Raman scattering study of Ba5RTi3Nb7O30 and Ba4R2Ti4Nb6O30 (R=Nd, Sm) provided rich complementary information for the structure change in phase transition. In relaxor of Ba5RTi3Nb703o, the intensity of some vibration modes increased under phase transition temperature, the wavenumber shifted gradually, suggesting the gradual structural change in relaxor ferroelectric. While for Ba4R2Ti4Nb6O30 (R= Nd、 Sm、Eu) with the first-order phase transition, some vibration peaks splitted up under the phase transition temperature, indicating the sudden change of the structural symmetry.