Defect Complexes Construction And Dielectric Properties Of ABO₃ Ceramics

ABO₃ perovskite ceramic is a kind of dielectric materials widely used in microelectronic devices.As the demand for miniaturization of electronic devices increases,ABO₃ structure ceramic needs to further improve the dielectric constant without increasing its dielectric loss.Constructing complex structures,which can make the space charges move at a certain range and confining the charges in the traps of complexes at the same time,is an effective method to increase the dielectric constant and suppress dielectric loss.In this paper,ABO₃ ferroelectric ceramic sodium bismuth titanium（NBT）and paraelectric ceramic strontium titanium（ST）are used as the based materials.The existence forms and forming conditions of the complex structures in the two ceramic systems are studied respectively.The effect and influence mechanism of complex structures on the dielectric properties are also investigated.X-ray diffraction,first-principles simulation and dielectric property measurement reveal the rules and mechanism of doping on the complex structures and dielectric properties of the ABO₃ ceramics.The study found that Ba doped NBT ceramics caused a dielectric anomaly peak behind Curie temperature at low frequency.The anomaly peak was most obvious when Ba donor substituting Na in NBT.This dielectric anomaly peak disappears at high frequency,exhibiting typical dipole moment characteristic.This indicates that Ba donor doping induces the localized ordering occupation of Bi and Na at the A-site of NBT,forming a dipole-type complex structure.The complex structure has little effect on the dielectric constant,but significantly suppresses the high-temperature dielectric loss of the ceramics.In(Ba_xBi_0.5Na_0.5-x)Ti_0.9Mg_0.1O₃ ceramics,with the increase of Ba content,the dielectric loss at high-temperature gradually decreases.When x=0.2,dielectric temperature stability of the ceramic sample is optimal.In the range of 200-400 ^oC,dielectric constant of the sample is maintained between 2500and 3500,and dielectric loss is less than 0.023.The temperature at which the dielectric loss increases sharply over 0.1 was postponed to 460 ^oC.First principle method was used to simulate the total energy of pure NBT and Ba-doped NBT supercells with different A-site ordering.It is found that the"001"ordered structure model had the lowest and most stable energy among all A-site ordered structure models.The formation energy of oxygen vacancy and the oxygen migration barrier energy in the Ba doped NBT model were calculated.The results showed that Ba doping inhibits the formation of oxygen vacancy and increases the migration barrier energy of oxygen ions.Then the conduction of oxygen ions at high temperature is inhibited,and finally the dielectric loss at high temperature is reduced.Sr_1-1.5xBi_xTiO₃ ceramics are prepared by presetting Sr deficiency,and the room temperature colossal permittivity property is successfully obtained by constructing defect complexes in doped ceramics.The results of dielectric property show that the sample with x=0.05 sintering at 1500 ^oC and after aging treatment has the best dielectric performance.Dielectric constant of this sample is up to 15000 in the full test frequency range,while the dielectric loss is maintained below 0.05 testing from10² to 10⁵Hz.Fine scan XRD results showed that the diffraction peak shifted to a lower Angle with the increase of doping concentration,and the Angle difference between（200）,（111）and（110）diffraction peaks increased with the increase of x,but the increase amplitude is different.Combining with first principle calculation results,it can be found that Bi^·_Sr-V_Sr^′′-Bi^·_Sr linear defect complex is formed in Sr_1-1.5xBi_xTiO₃ ceramics.This defect complex“pinned”hole charge which leads the Sr_1-1.5xBi_xTiO₃ceramics to possess excellent giant dielectric constant with low dielectric loss.The frequency dependent dielectric constant of non-stoichiometric Sr_1-xBi_xTiO₃ceramics shows that an excellent colossal dielectric property with a dielectric constant of 30,000 and a dielectric loss below 0.03 is obtained at x=5%.By analyzing the XRD and XPS results,it is found that Sr_1-xBi_xTiO₃ ceramics have Bi anti-occupancy structure.First principles calculation results revealed that the anti-occupancy Bi and oxygen vacancies in non-stoichiometric Sr_1-xBi_xTiO₃ ceramics formed the Bi^′_Ti-V_O^··-Bi^′_Ti defect complex.The donor electrons are“pinned”in the Bi^′_Ti-V_O^··-Bi^′_Ti defect structure wihch resulting in Sr_1-xBi_xTiO₃ ceramics to have excellent giant permittivity property.