Dielectric Characteristics And Ferroelectric Transition Of Zr-Based Filled Tungsten Bronze Ceramics

As the important electronic materials,ferroelectrics have wide applications in piezoelectric,pyroelectric,optoelectric and nonlinear optic devices.Tungsten bronze oxides constitute the second largest classes of dielectric materials just next to perovskites,and they have unique dielectric characteristics and rich physical meaning.In the present thesis,the crystal structure,dielectric characteristics and ferroelectric transition have been investigated systematically,and the crossover from normal ferroelectric to diffuse and relaxor ferroelectric has been revealed.Ba4R2Zr4Nb6O30(R=La,Nd,Sm)with filled tetragonal tungsten bronze structure has been synthesized and investigated.The present materials show a crossover from relaxor to normal ferroelectric feature with increasing the.A-site ionic radii difference(Δr).The relaxor behavior in Ba4La2Zr4Nb6O30 is associated with the incommensurate structure modulation,while the commensurate structural modulation dominates the normal ferroelectric transition in Ba4Nd2Zr4Nb6O30 and Ba4Sm2Zr4Nb6O30.Ferroelectric transition in Ba4Nd2Zr4Nb6O30 and Ba4Sm2Zr4Nb6O30 shows the first order feature with unusual large thermal hysteresis(>100K),and it is followed by a low temperature dielectric relaxation.Abnormal decrease of remanent polarization is observed at lower temperature for Ba4Nd2Zr4Nb6O30 and Ba4Sm2Zr4Nb6O30,which is attributed to the disruption of the ferroelectric polar order by the micro polar regions appearing at the same temperature region.Tetragonal tungsten bronze structure with space group P4/mbm is determined for Ba5RZr3Nb7O30(R＝La,Nd,Sm),ion cross distribution by Ba2+ and R3+ in A1 site is observed,while A2 site is only occupied by Ba2+.Selected area electron diffraction patterns confirm the existence of incommensurate superlattice modulation.Furthermore,temperature and frequency dependences of the dielectric properties show a broad permittivity peak with strong frequency dispersion,following well the Vogel-Fulcher relationship.The dielectric maximum temperature,which is related to the ferroelectric transition,increases gradually with decreasing the Al site ion size.Slim P-E hysteresis loops are obtained at room temperature for all compositions.The cross distribution at A1 site and Zr substitution at B site is responsible for the enhanced relaxor nature.Except A site ions occupations,B site ions substitutions also play an important role in the ferroelectric property of tetragonal tungsten bronzes,such as the relaxor to normal ferroelectric crossover in Ba4Sm2Zr4(NbxTa1-x)6O30(x=0,0.3,0.5,0.7,0.9,1)ceramics.The tetragonal tungsten bronze structure in space group P4/mbm is confirmed for compositions of x=0,0.3 and 0.5,while space group P4bm is confirmed for compositions of x=0.7,0.9 and 1.With increasing Nb content in B site,the dielectric properties show a crossover from relaxor to normal ferroelectric behavior,accompanied with the disappearance of frequency dispersion,increased ferroelectric transition temperature and larger dielectric constant value.The room temperature Raman spectra for compositions of x=0 and x=1 show different vibration modes of the oxygen octahedral,and the temperature dependence of the integrated intensity of Raman bands indicates the different structural transition process.The onset of relaxor/normal ferroelectric behavior of Ba4Sm=Zr4(NbxTa1-x)6O30 is mainly caused by the different electron configuration of Nb and Ta.Ferroelectric transitions of Ba4Sm2(Ti1-xZrx)4Ta6O30 are also dominated by the B site substitutions.For x=0 and x=0.1,the tetragonal tungsten bronze structure with space group P4bm is determined.A first order ferroelectric transition with a sharp,frequency independent dielectric permittivity maximum is indicated for x=0 and x=0.1,while a strong low temperature dielectric relaxation is also determined for x=0.1.For x=0.3,0.5,0.7 and 1,a tetragonal tungsten bronze structure with space group P4/mbm is determined.Relaxor ferroelectric behavior with strong frequency dispersion becomes dominating for compositions of x=0.3,0.5,0.7 and 1,and they exhibit similar transition temperature and diffuse degree.Compositions of x=0 and 0.1 exhibit nonlinear polarization electric-field(P-E)hysteresis loop at room temperature,while other compositions only exhibit linear curves.Raman spectra present distinct vibration modes for x=0 and x=1,and the intensity of some modes show sudden rising or falling with increasing x from x = 0 to x= 0.1.Both the different electron configuration and ion size mismatch between Zr and Ti lead to the normal ferroelectric to relaxor crossover.The aging effect and metastable ferroelectric state have been investigated in Ba4Eu2(Ti0.9Zr0.1)4Ta6O30 ceramic,where the tetragonal tungsten bronze structure with P4bm space group is confirmed.When the compound is cooled from temperature far above its transition temperature(TC),a very slim polarization-electric field(P-E)curve is observed at 300 K.After three days aging at 300 K,an obvious increase of the remanent polarization and coercive electric field can be obtained.Moreover,a gradual recovery of the endothermic peak is detected in the differential scanning calorimeter(DSC)curves when the compound is cooled from temperature above Tc and aged at 300 K for several days.These phenomena are interpreted as the results of a metastable ferroelectric state and the related aging effect,which may be deeply concerned with the compositional inhomogeneity and the hindered superstructure transformation.