Study Of The Lattice Defects Of Perovskite Ferroelectric Materials On The Ferroelectric And Optical Absorption Properties

Ferroelectric materials have many applications in contemporary society,such as piezoelectric sensors,actuator,transducer,photovoltaic,photocatalytic and so on.High performance ferroelectric materials are very important and widely used.It is also one of the hot topics in advanced materials research.In order to maintain the sustainable development of the society,many countries have made a lot of efforts to study the new type of lead-free piezoelectric ceramics.While defects are very important for piezoelectric materials,they have both good and bad sides.In this paper,effects of the defects on the piezoelectric,dielectric,photoelectric and photocatalytic properties of the materials are studied in four aspects.?I?the study on the occupying position and properties of Li₂CO₃ doped BaTiO₃;?II?the study on the occupying position and properties of LiF doped BaTiO₃;?III?the study on optical absorption of NiO and Nb₂O₅co-doped BaTiO₃;?IV?the study on photocatalytic activity of La₂O₃ doped SrBi₂Nb₂O₉ferroelectric materials.We prepared three kinds of Li⁺doped BaTiO₃ ceramics by the solid state reaction method:?I?(Ba_1-xLi_x)TiO_3-x/2 having A-site Li⁺,?II?Ba(Ti_1-xLi_x)O_3-3x/2 having B-site Li⁺,and?III?x/2 Li₂CO₃+BaTiO₃ mixed one,for which we investigated the stable site of Li⁺ions.The density of all prepared ceramics are above 95%.The results show that the lattice structure,the grain size,and the electric properties of Li⁺ions doped BaTiO₃ceramics are dependent on Li⁺ions site.According to increasing of Li⁺content,the cell volume of Ba_1-x-x Li_xTiO_3-x/2-x/2 decreases,but that of BaTi_1-xLi_xO_3-3x/2 increases.That of x/2Li₂CO₃+BaTiO₃ decreases by the small addition of Li⁺ions,but increases by the large addition of Li⁺ions.All Li⁺ions doped ceramics show antiferroelectric-like double hysteresis loops.By comparison with the results of three type ceramics,it is concluded that at x/2Li₂CO₃+BaTiO₃ ceramics,the Li⁺ions prefers to favorably substitute Ba²⁺at A site for the low concentration of Li⁺ions but its location was changed to Ti⁴⁺-site for the high concentration of Li⁺ions.We prepare three different LiF doped BaTiO₃ ceramics by solid state reaction,i.e.,?I?the Ba_1-x-x Li_xTiO_3-x-x F_x ceramics?abbreviated as A:BLiFT-x?,?II?the BaTi_1-xLi_xO_3-2xF_x ceramics?abbreviated as B:BTLiF-x?,and?III?the xLiF+BaTiO₃ mixed ceramics?abbreviated as C:LiFBT-x?.Through XRD analysis and electrical properties comparison,it is concluded that Li⁺ions occupies A site and F^-ions occupies O site in A:BLiFT-x and C:LiFBT-x ceramics,but the solubility of LiF is small and most occupy the interstitial site.With the increase of LiF,the second phase appears in the ceramics due to the solubility.Thus there are two peaks near Curie point.The value of d₃₃ and k_p increase first and then decrease with the increase of LiF content.However,Li⁺ions occupies B-site in B:BTLiF-x ceramics.The[Li _Ti-F_O]^2-defect dipole is formed and a thin waist hysteresis loop appears.The d₃₃ and k_p decrease with the increase of LiF content.For all three group of ceramics,the Curie temperature decrease and the dielectric constant increase with the increase of LiF content.BaTi_1-x[Ni_1/2Nb_1/2]_xO_3-??x=0%,1%,2%,3%,4%,5%??BTNN-x?ceramics prepared by the solid state method showed strong visible light?VL?absorption with enhanced intensity by increasing co-doping of NiO and Nb₂O₅.Surprisingly the absorption was significantly removed in the powders produced from the ceramics.It indicates that the visible absorption of ceramics is not the intrinsic property of BaTi_1-x[Ni_1/2Nb_1/2]_xO_3-?.Therefore,we suggest that the role of the grain boundary of ceramics for the VL absorption should be important.The complex structure of impurities coupled with defects and the disordered structure at grain boundary should be engineered for the application.The first-principles calculations indicated that deep level defects are responsible for the visible absorption.This conclusion is also the first discovery in our group.The effect of La₂O₃ doping on the efficiency of the photocatalysis of SrBi_2-xLa_xNb₂O₉ ferroelectric powders and the ceramics synthesized by the solid state method is investigated.It is found that the photocatalysis efficiency is not always correlated to the polarization.Raman spectra reveals that the microscopic structure of SrBi_2-xLa_xNb₂O₉ depends on the content?x?of La³⁺ions,and the La³⁺ions occupy mainly the Bi-sites in the Bi₂O₂ layers for the low x,whereas for the higher x,they can also replace Sr²⁺ions in the A sites of the perovskite blocks?SrNb₂O₇?.Additionally,the spontaneous polarization increases with the increase of La³⁺content.For the low x?x?0.02?,the leakage current decreased with La³⁺content and the photocurrent and the photocatalysis?PC?efficiency increased with La³⁺content,due to the reduction of O-vacancy,whereas for the higher x?x>0.02?,the leakage current increases with x due to the formation of donor state La⁺_Sr,and the photocurrent and the PC efficiency decreased with x due to the increased scattering rate by La⁺_Sr,although the spontaneous polarization increases continuously with the La³⁺doping,indicating that the role of the defects such as La⁺_Sr impurity states for the carrier transport plays the more important role than the spontaneous polarization.