Studies On Bandgap Narrowing And Properties Of Ti-Based Ferroelectrics

Titanium-based perovskite ferroelectrics have gained a place in multifunctional materials due to the non-toxic,stable structure and outstanding performance.They have simple structures and show the large doping tolerance.The ion-doping at the specific lattice place can induce the structural transformation,the narrow band gap and the changed electronic spin state.Ferroelectrics also can compound with a variety of oxides,resulting in multifunctional properties of the materials.After the performance optimization using different methods,the materials show wide application fields including low-power information storage devices,spintronic devices and photoelectric energy converters.With the development trend of the expanding energy demand and the optimized energy structure,the research of the ferroelectric photovoltaic effect has become a hot area of current attention.However,the most of ferroelectric oxides have the wide band gaps,resulting in the utilization of solar spectrum concentrated on the part of high frequency bands,which seriously limits the photoelectric conversion efficiency of ferroelectric photovoltaic devices.Therefore,it is important to explore ferroelectrics with the narrow band gap for high-performance devices.In this paper,the titanium-based ferroelectric oxides and related derivatives,Sr_0.3Ba_0.7Ti O₃(SBTO)?Bi_0.5Na_0.5Ti O₃(BNTO)?Ba_0.85Ca_0.15Zr_0.1Ti_0.9O₃(BCZTO)and Ba Ti O₃(BTO),have been prepared with the different methods and systematically explored.The magnetic,optical,electrical behaviors and the corresponding mechanisms have been analyzed,which provides a new design idea for the study of high-performance materials and devices.The innovative results of this dissertation include the following points:1.The compositional effects of spin magnetic orders and the optical response for Sr0.3Ba0.7Ti1-_xPd_xO3-?(xSBTPO)films are analyzed by testing methods.SBTO films have been synthesized on quartz substrate by sol-gel method,and the optimum annealing temperature has been investigated.In addition,Pd²⁺doped xSBTPO thin films have been synthesized to achieve the regulation of ion doping on the structure and properties of SBTO thin films.XRD data indicate that the optimum annealing temperature of SBTO film is 800?,and xSBTPO films exhibit the good crystallinity.The introduction of transition metals leads to the transformation of SBTO film from cubic phase to tetragonal phase,and the difference of ionic radius leads to the increased lattice strain.SEM images show that xSBTPO film is relatively compact and smooth as a whole,and the grain size decreases gradually with the increased doping concentrations.This behavior is caused by the difference in bond energy between Pd-O bond and Ti-O bond.The formation of Pd-O bond is not conducive to the growth of grain,resulting in the slowing down of grain growth rate and the reduction of grain size.The optical transmission analysis shows that the fundamental absorption edges appear the red shift,which reflects that the band gap of the sample decreases gradually.The d orbital energy level splitting of Pd²⁺into between the conduction band and valence band of SBTO crystals is responsible for the narrow band gap.The hysteresis loop of 0.09SBTPO film has been done at RT,and the ferromagnetic curve shape is relatively standard.The M_s,M_r and H_c of 0.09SBTPO film are 6 memu/cm~3,0.8 memu/cm~3 and 120 Oe,respectively.This behavior results from the lattice distortion attributed to the increased internal stress of the lattice,which induces the increase of the spin tilt angle and changes the antiferromagnetic spiral structure,and thus produces ferromagnetic behaviors.2.The band-gap narrowing,RT ferroelectric and ferromagnetic of Bi0.5Na0.5Ti1-_xCo_xO3-?(xBNTCO)ceramics have been realized by solid state reaction method.In addition,Bi0.46Na0.46Ba0.08Ti1-_xCo_xO3-?(x BNBTCO)ceramics have been prepared.The various behaviors caused by transition metal ions on the morphology,structural transformation and optical response of ceramic crystals have been studied.XRD images and Raman scattering analysis show that xBNTCO/x BNBTCO ceramics have the perovskite structure without other impurity phases,but the crystal symmetry changes and the transition from trigonal phase to pseud-cubic phase.XPS data indicate the presence of lattice oxygen and oxygen vacancies in the 0.02BNTCO ceramic,which is caused by the loss of oxygen as a gas.The introduction of Co²⁺ions increases the absorption and utilization of the optical spectrum.The band-gaps of xBNTCO/x BNBTCO ceramics reducd to 1.87 e V and 1.3 e V,respectively.The obvious decrease of optical band gap can be explained that O-vacancy induces new electron orbital energy levels between the CB and the VB,which takes part in the electron transition of xBNTCO/x BNBTCO ceramics,resulting in the decreased band-gaps and the appearance of partial absorption peaks.In xBNTCO ceramics,the ferromagnetic and ferroelectric properties of the crystals are optimized by ion-doping at RT.The magnetic behaviors change from diamagnetism to ferromagnetism and then to para-magnetism,which can be well illustrated using F center exchange theory.The0.04BNTCO ceramic exhibits good ferroelectric properties,and the P_r is 11.47?C/cm~2.In order to clearly understand the reasons of the increased polarization,the ionic displacement model is constructed to explain the influence of oxygen vacancies on the crystal structure and ionic displacement.Because of the further increased doping concentrations,the contents of oxygen vacancy caused by ion substitution increase continuously.The aggregation of oxygen vacancy at the domain wall leads to the obstruction of crystal domain exchange,which is attributed to the reduction of ferroelectric polarization.3.The synthesis of(1-x)Ba Ti O3-x Ba Nb1/3Cr2/3O3-?(xBTNCrO)and(1-x)Ba Ti O3-x Ba M0.5Nb0.5O3-?(x BTNMO,M=Ni/Co)ceramics and the study of physical properties.The effects of transition metal elements on the phase transformation,the grain size,the band-gap narrowing,the ferroelectric and ferromagnetic properties at RT have been analyzed by various testing methods.The XRD results show that xBTNCrO/x BTNMO ceramics have a single crystal structure and find no other impurity phases.Due to the increase of impurity concentrations,the parent material changes from tetragonal structure to cubic structure.The introduction of transition ions with larger radius causes the changed lattice strain and the gradually larger cell volume.Raman scattering patterns confirm the existence of phase transition and exhibit a decrease in the local lattice symmetry of the material,which is inconsistent with the ideal cubic with Pm3m space group.The grain size has been measured and it can be found that the grain size decreased significantly with the increase of doping concentrations.This phenomenon is attributed to that the doping ions are located near the grain boundary,which reduces the speed of crystal boundary movements and the rate of the crystal growth.The optical absorption spectrum shows that the absorption range of the doped materials expands and the corresponding band-gap decreases to a certain extent.The wide band-gap determined by O 2p orbitals and Ti 3d orbitals of original material(BTO)is 3.1 e V.The band-gaps of 0.2BTNCo O ceramics and 0.15BTNCr O ceramics are 1.19 e V and 2.1 e V,respectively.The band-gap narrowing is explained from the perspective of energy band structure.The transition metal elements Cr and Co have 3d orbits,and the 3d orbits are divided into multiple degenerate orbits with different energy levels under the effect of the crystal field,which are situated between the conduction band and valence band.Hence,this reduces the band-gap of the materials.The ferroelectric test shows that BTO has good ferroelectric polarization.However,the addition of impurities is not conducive to the ferroelectric properties of xBTNCrO and x BTNMO ceramics,and the appearance of leakage current caused by defects such as oxygen vacancy is responsible for the opening in the curve.The perfect S-shaped curve appears in xBTNNO ceramics,indicating the establishment of ferromagnetic long-range orders.According to the magnetic theory,the magnetism of materials is attributed to the exchange of conduction electrons and spinning electrons.In the long range exchange process,the conduction electrons induce the adjacent Ni²⁺ions to produce the same magnetization direction,so the magnetic moments of Ni²⁺ions have the same direction,showing ferromagnetic behaviors at the macro level.4.Preparation of transition metal doped Ba0.85Ca0.15Zr0.1Ti0.9O3 ceramics and the study on optical,electric and magnetic properties.Ba0.85Ca0.15Cr_xZr(1-_x)/10Ti9(1-_x)/10O3-?(x BCCZTO)ceramics with Cr³⁺ions have been synthesized.The structural transition,molecular vibration modes and the surface morphology of the materials have been observed.The internal stress and crystal plane spacing have been analyzed based on the test data.The band-gap narrowing and ferroelectric behaviors can be controlled by the introduction of precise doping concentrations.x BCCZTO ceramics have been prepared using low cost oxides and carbonate as initial materials by the solid phase reaction technique.The Williamson-Hall analysis method is used to calculate the internal lattice strain values of the all samples.With the increase of the Cr³⁺concentrations,the tensile stress in the crystal gradually decreases along with the expansion of crystal plane spacing,reflecting the appearance of crystal lattice distortion.The composition of BCZTO ceramics is detected by EDX,and the proportion of elements is consistent with the theoretical value.The sum of A-sites elements is slightly smaller than the sum of B-sites elements,reflecting the existence of A-sites defects in the crystal.The absorption range of materials with Cr³⁺ions is obviously enlarged,especially in the wavelength range from 400 nm to 800 nm.The band-gap decreases from 2.95 e V to 1.2 e V.P-E loops show that 0.02BCCZTO ceramic has ferroelectric properties and the spontaneous polarization intensity is the largest,indicating that appropriate ion doping can improve the ferroelectric behavior of x BCCZTO ceramics.This behavior is attributed to the movement of A-site and B-site ions caused by the appearance of oxygen vacancy,which results in the change of positive and negative charge centers.The hysteresis loop shows that the curve shape of 0.06BCCZTO component is standard at RT,showing the traditional"S"shape,inducing the establishment of long-range ferromagnetic orders.