Structure, Dielectric Characteristics And Ferroelectric Transition Of Filled Tungsten Bronze Niobates

In the present thesis, the structures, dielectric characteristics and ferroelectric transition were systematically investigated for the Sr-based filled tungsten bronze niobate ceramics, the effects of A-site cations upon on the structure and characteristics were especially emphasized, and the structure origins of the ferroelectric transition and the dielectric relaxations were revealed.In the Sr4R2Ti4Nb6O30 system, compounds with R=La, Nd, Sm and Eu formed the tetragonal tungsten bronze structure in space group P4bm, while Sr4Y2Ti4Nb6O30 could not form the stable tetragonal tungsten bronze structure. This indicated that the stability of the filled tungsten bronze structure decreased with decreasing R cation radius. The structure refinements indicated that the off-center displacement of Ti/Nb cations in both oxygen octahedra along the c axis in compounds with R=La & Nd. [Ti/Nb(2)]O(3)6 had much severer distortion than [Ti/Nb(1)]O(1)6, and the oxygen distortion in Sr4Nd2Ti4Nb6O30 was severer. With the decreasing R cation radius, the ferroelectric transition in Sr4R2Ti4Nb6O30 changed from relaxor type to diffuse transition, then to normal ferroelectric transition. Sr4La2Ti4Nb6O30 had a relaxor ferroelectric transition around 200 K. The (f, Tm) data well obeyed the Vogel-Fulcher relationship with Tf=165.1 K, Ea=0.104 eV and fo= 2.06×1014 Hz, which suggested that the relaxor behavior was analogous to a spin glass with polarization fluctuations above a static freezing temperature. Sr4Nd2Ti4Nb6O30 had a diffuse ferroelectric peak at 500 K without frequency dispersion. Sr4Sm2Ti4Nb6O30 and Sr4Eu2Ti4Nb6O30 were normal ferroelectrics with the first-order ferroelectric phase transitions at 606 K and 601 K, respectively. The enthalpy of the transitions△Htrs were 1.502 kJ/mol for R=Sm and 2.263 kJ/mol for R=Eu. Moreover, the ferroelectric transition indicated large thermal hysteresis for both ceramics, where the ferroelectric transition peak disappeared during the cooling process. The large thermal hysteresis was attributed to the complex crystal structure of the tungsten bronze. Besides the ferroelectric transition peak, a low temperature relaxation was observed in all compositions, and was attributed to the polarization in the ab plane originated from the tilting and distortion of oxygen octahedra.Four dielectric anomalies were observed in the Sr4(LaxNd1-x)2Ti4Nb6O30(x=0, 0.25,0.5,0.75,1) system. With increasing x value, the high-temperature diffuse ferroelectric peak gradually changed into the low-temperature relaxor ferroelectric peak, accompanied by another dielectric relaxation at the temperature higher than the relaxor ferroelectric transition temperature. This relaxation had the largest intensity when x=0.5, and was associated with the cation distribution in the Al site. The relaxation caused by the polarization in the ab plane existed in the lowest temperature range. All the dielectric relaxation peaks could be fitted by the Vogel-Fulcher relationship. The relaxation caused by the polarization in the ab plane had larger active energy Ea (0.15 eV-0.2 eV), while the micro polar region originated from the c axis polarization (the relaxor ferroelectric peak) had smaller Ea (about 0.1 eV).In the (BaxSr1-x)4Nd2Ti4Nb6O30 (x=0,0.2,0.4,0.6,1) system, the diffuse ferroelectric peak changed into the normal ferroelectric peak gradually with increasing Ba content. Moreover, two dielectric relaxations existed below the ferroelectric transition peak, and were associated with the cation distribution in A2 site and the micro polar region in the ab plane. The above results indicated that the ferroelectric transition in the M4R2Ti4Nb6O30 tungsten bronzes was dominated by both the A1 and the A2 site cations. Tungsten bronzes with larger radius difference between the A1 and the A2 site cations (△R=RA2-RA1) tended to have normal ferroelectric transition, while Tungsten bronzes with smaller radius difference tended to be relaxor ferroelectrics.Crystal structure, dielectric properties and the ferroelectric transition were investigated in the Sr5RTi3Nb7O30 (R=La, Nd, Sm, Eu & Y) system. In Sr5RTi3Nb7O30 (R=La, Nd, Sm, Eu), the relation between the ferroelectric transition and△R was similar to that of M4R2Ti4Nb603o. Sr5YTi3Nb703o was quite abnormal, which had similar dielectric behavior to Sr5NdTi3Nb7O30. Sr5LaTi3Nb7O30 performed as relaxor ferroelectrics with the relaxor ferroelectric transition peak increasing from 235.4 K to 266.1 K with increasing frequency from 100 Hz to 1 MHz. Sr5NdTi3Nb703o had the diffuse ferroelectric transition peak around 435 K. The normal ferroelectric transition appeared at 530 K for Sr5SmTiNb7O30 and 540 K for Sr5EuTi3Nb7O30 with large enthalpy of 7.009 kJ/mol and 5.263 kJ/mol, respectively. Dielectric relaxation existed in all the compositions below the phase transition temperature, and became weaker with increase of the R cation radius. This relaxation was attributied to the freeze-out of the polarization in the ab plane. The dielectric properties of Sr5YTi3Nb703o were quite similar to Sr5NdTi3Nb703o, which did not follow the above trend. Structure refinement results indicated that, the distortion of oxygen octahedra become severer with decreasing R cation radius, and the smaller R cation had larger mobility in the A1 site, which should benefit the normal ferroelectric transition. However the off-center displacement of Ti/Nb cations in the ab plane became weaker, corresponding to the decrease of the low temperature dielectric relaxation.Raman scattering and ultrasonic study of Ba4Nd2Ti4Nb6O30 provided rich complementary information on the structure change and dielectric relaxations. The step-like anomalies in the wave number of Raman vibrations as well as the abnormal drop of intensity of Raman spectra were observed around and above Tc, which proved the first-order ferroelectric phase transition. Low temperature structure fluctuations were detected by the ultrasonic anomaly at about 120 K and the increasing of Raman intensity below 150 K.