Multiferroic Niobate Ceramics With Tetragonal Tungsten Bronze Structure

Multiferroic materials have attracted extensive attention due to their special physical properties and potential application in new multifunctional devices.And multiferroic materials have become one of the most important research hotspots in condensed matter physics and material science.Due to its abundant cations and tunable structures,the multiferroic tetragonal tungsten bronze structure niobate ceramics have great potential in the field of multiferroics.But some key scientific questions remain:what is the source of ferroniobate’s magnetism;the relationship between the ferroelectric/relaxation ferroelectric properties and the commensurate/incommensurate oxygen octahedron tilting modulation needs to be verified;the effects and mechanisms of different cationic positions on the structure and properties;the effects of oxygen vacancies on its commensurate/incommensurate modulation and ferroelectric properties have not been systematically studied and analyzed.In this dissertation,the effects of ion displacement and oxygen vacancies on the structure and properties of multiferroic niobate ceramics with tetragonal tungsten bronze structure are systematically studied.The following conclusions are obtained:In Ba4(EuxLa1-x)2Fe2Nb8O30(x=0,0.3,0.5,0.7,0.9,0.95,1)ceramic system,with the increase of Eu concentration,the average ionic radius of A1 position of the tungsten bronze decrease gradually decreases,and thestructure gradually transforms to the commensurate modulation structure.For compositions of x=0.9,0.95,a first order ferroelectric transition can be observed above room temperature,and saturated P-E electric hysteresis loops can be detected at room temperature,coupling well with commensurate oxygen octahedral tilting modulation.Commensurate oxygen octahedral tilting modulation can promote ferroelectric transition.All compositions are ferromagnetic at room temperature,which can be ascribed to tungsten bronze structure itself and the iron rich of the second phase.In(SrxBa1-x)4Sm2Fe2Nb8O30(x = 0,0.1,0.1,0.2,0.4)ceramic system,composition of x=0 shows an obvious ferroelectric abnormality which indicates the first order ferroelectric phase transition.While for compositions of =0.1,0.2,0.3,0.4,only a wide dielectric abnormal peak is detected,which indicates relaxor ferroelectric properties.With the decrease of the ionic radius difference between A1 and A2 sites,(SrxBa1-x)4Sm2Fe2Nb8O30 gradually transforms from ferroelectric to paraeleltric.Typical ferroelectric hysteresis loops are detected in compositions of x=0 and 0.1 above room temperature.And with the increase of the Sr ions,the ferromagnetic property enhances.Therefore,the larger average ion radius of A2 position is conducive to the generation of ferroelectric properties of(SrxBa1-x)4Sm2Fe2Nb8O30 tungsten bronze structural ceramics,and the introduction of Sr ions is conducive to the enhancement of magnetic properties.In Ba4Sm2FexTi4-2xNb6+xO30(x = 0,0.5,1,0.5,2)ceramic system,first order ferroelectric phase transition above room temperature can be observed in all compositions.With decrease of Ti,Curie temperature(Tc)shift to low temperature.Remanent polarization detected by PUND method confirms the ferroelectric property.With the increasing x,coercive field(Ec)decreases and ferromagnetic property enhances obviously.These results show that in tetragonal tungsten bronze structure,cations in B site plays an important role on ferroelectric and magnetic property.And in composition of x=1 the single-phase multiferroic materials at room temperature is achieved.In multiferroic Ba4Eu2Fe2Nb8O30 ceramics with tungsten bronze structure,maximum polarization of samples sintered in air decreases from 4.0 μC/cm2 to 3.6μC/cm2 after annealed in oxygen atmosphere;maximum polarization of samples sintered in oxygen atmosphere increases from 3.5 μC/cm2 to 7.0 μC/cm2 after annealed in nitrogen atmosphere.The remanent polarization obtained from PUND method exhibits a reversible enhancement,which is confirmed to be asscociateed with the commensurate oxygen octahedral tilting modulation induced by oxygen vacancies.The results of TEM analysis indicate that the increase of concentration of oxygen vacancies promote the commensurate octahedral tilting modulation,which enhances the ferroelectricity.In multiferroic Ba4Nd2Fe2Nb8O30 ceramics with tungsten bronze structure,samples sintered in air transforms from saturated P-E loop to linear P-E loop after annealed in oxygen atmosphere;samples sintered in oxygen atmosphere transforms from linear P-E loop to saturated P-E loop after annealed in nitrogen atmosphere.The remanent polarization obtained from PUND method exhibits an enhancement with the increasing oxygen vacancies.The remanent polarization of samples sintered in air decreases from 0.32 μC/cm2 to 0 μC/cm2 after annealed in oxygen atmosphere;the remanent polarization of samples sintered in oxygen atmosphere increases from 0μC/cm2 to 0.92 μC/cm2 after annealed in nitrogen atmosphere.Similar with Ba4Eu2Fe2Nb8O30 ceramic system,ferroelectric polar:ization enhancement is also proved to be coupling with the commensurate oxygen octahedral tilting modulation induced by oxygen vacancies.And this enhancement induced by oxygen vacancies is reversible,providing a new idea for looking for new types of strong ferroelectric room temperature multiferroic materials.