Structures And Dielectric Properties Of Rare Earth And Rare Earth/Mn Doped BaTiO₃ Ceramics

BaTiO₃ ceramics possess superior dielectric and piezoelectricity properties,and are important in the field of electronic ceramics.The Curie temperature?T_c?is too high and the variation of dielectric constant around T_c is large for pure BaTiO₃,which inhibit the application.Doping is widely used to improve the dielectric properties of BaTiO₃ with temperature stability.However,the valences,site occupation and compensation mechanisms are sensitively affected by ionic radii,crystal structures and prepare conditions,which make it difficult to identify the defect states and the influence on the electric properties.Although doping is acknowledged as an effective method for the enhancing the properties of BaTiO₃,the interpretation about the doping mechanisms is still in dispute.Therefore,more work need to be carried out to explore high performance BaTiO₃ ceramics and the relation between the structure,defect and properties.In this work,we prepared doped-BaTiO₃ ceramics with different elements and compensation mechanisms by solid-state reaction method,and studied the structures,morphologies,valence states and dielectric properties by means of XRD,Raman spectroscopy,SEM,electron paramagnetic resonance?EPR?and dielectric spectrometer,and The main contents listed as following:1.We prepared Nd-doped BaTiO₃ ceramics,and Ti vacancies were introduced to compensate the donor Nd ions.All samples have perovskite structure with single phase,indicating the Nd ions completely entered the BaTiO₃ lattice.The room-temperature structure changes from tetragonal to cubic with doping content increasing.The samples exhibit colossal dielectric constant and dielectric relaxations at low temperature,and new dielectric constant plateau and relaxation peaks appeare at higher doping content.The relaxation peaks shift towards higher frequencies with increasing temperature,showing a thermal activation process,and the activation energy decreases with increasing doping content.The analysis of impedance and electric modulus suggests that the responses of grain boundary with high resistivity,grain and grain core with high conductivity exist at low,middle and high frequencies,respectively,and the grain core influence on the dielectric constant could be neglected.The composition and electric properties of grain become inhomogeneous in highly doped samples,and more responses appear in impedance and electric modulus spectra.The charge carriers are localized,which is associated with the formation of defect complexes.The defect complexes and microstructure both influence the formation of electric dipoles and the distribution of space charge.The high dielectric constant at low and intermediate frequencies is the result of space charge and dipole polarizations.2.Nd/Mn codoped BaTiO₃ ceramics were prepared,the Mn content was 3%and Nd content was 1%,2%,4%and 6%,respectively.The ceramics have a relative density as high as 92%.With the doping content increasing,the crystal structure changes from a mixture of hexagonal and tetragonal to single tetragonal,and then to a cubic phase.T_c shifts towards room temperature and the dielectric constant at T_c is enhanced with increasing doping content.The sample x=0.06 possesses the best dielectric performance and satisfies the Y5V specification,and the loss is less than0.04.The EPR results indicate that Nd³⁺ions enable the valence state of Mn ions to change from high valence to low valence state,and all of the Mn ions are divalent in x=0.06,which could be ascribed to the formation of2Nd^?_Ba-Mn'_Ti.The nonlinear decrease of cell volume and T_c with increasing doping content is also related to the defect complexes.3.Sm doped BaTiO₃ ceramics with the formula of Ba_1–xSm_xTi_1–x/4O₃?x=0.03 and0.06?have a tetragonal structure,and the tetragonality of x=0.06 is lower than x=0.03,suggesting that high doping content is favorable for the cubic phase.T_c and orthogonal-tetragonal transition temperature decrease with increasing doping content,and the descending rates are 15°C/at%and 20°C/at%,respectively.The colossal dielectric constant at low frequencies increases with doping content,which is a result of interfacial polarization.The impedance spectra indicate that the samples have a inhomogeneous electrical structure,and the outer grain region has larger resistivity,with is related to the formation of defect complex of Sm³⁺and Ti vacancies.The dielectric constant and relaxation at high frequencies is associated with the grain response,and the activation energy varies with temperature,which is related to the evolution of crystal structure.4.The Sm/Mn codoped BaTiO₃ ceramics(Ba_1–xSm_x)(Ti_0.99Mn_0.01)O₃?x=0.02⁰.07?have a relative density of about 93%.As the doping content increases,the crystal structure changes from tetragonal to cubic,and the cell volume decreases gradually,confirming that Sm³⁺occupies the Ba sites,and Raman analysis also support this conclusion.The EPR measurement at various temperatures suggests that Mn ions have mixed valence states of+2 and+4 in tetragonal phase,and only+2 valence is observed in cubic structure.The defect complex of2Sm^?_Ba-Mn'_Tican prevent Mn²⁺from being oxidized to higher valences and results in the quenching of high wavenumber fluorescence effect in Raman spectra excited by 532 nm laser.With the doping content increasing and the evolution of valence state of Mn ions,the dielectric performance is improved,and x=0.07 has the highest dielectric constant(??_m=15220,??_RT=13810)and low loss?tan?<0.03?.The dielectric constant is nearly frequency independent in the range of 1¹0⁶ Hz,indicating that ionic displacement polarization is dominant,whereas the space charge polarization could be neglected.