Studies On Synthesis And Properties Of Narrow Bandgap Fe-Based Perovskite Oxides

As a kind of important functional materials,ABO3-type perovskite ferroelectric shows a simple structure with larger doping tolerance.Different electronic configuration of A and B sites ions provide abundant combination types,which can induce different interactions between ions.These materials could be widely used in functional devices,which has become a frontier and hotspot of condensed matter physics and materials research.With the development of science and technology,the environmental and energy problems have attracted much attention.For the past few years,a lot of research is located on application of ferroelectric photovoltaic effect.However,most of the perovskite oxides are wide band gap insulators,which result in low utilization rate of solar spectrum.It hinders the improvement of photovoltaic conversion efficiency.Therefore,exploring narrow Eg ferroelectrics as the absorption layer of solar cell devices possesses significant meanings.Following the theory and experiment research methods,chemical modification on simple Fe-based perovskite oxide is studied firstly.And then,our work is dedicated to the research on band gap regulation behavior of A(Fe0.5Nb0.5)O3 double perovskite,according to BiFeO3(BFO),Pb(Zr,Ti)O3(PZT),KNbO3(KNO),BaTiO3(BTO),and other classic ferroelectrics.Besides,the phase transition and physical properties of these solid solution are systematically analyzed.Finally,the Ba(Fe0.5Nb0.5)O3(BFNO)double perovskite single crystal thin films are prepared by the pulsed laser deposition method,which provides a new idea for the development of high-efficiency photovoltaic devices.Our innovative results are listed as follows:1.Ceramics and thin films of Bi0.9Sm0.1FeO3(BSF),Bi0.9Sm0.1Fe0.95Mn0.05O3(BSFM)and Bi0.9Sm0.1Fe0.9Mn0.05Ni0.05O3(BSFMN)are prepared by solid state reaction method and sol-gel method,respectively.The influence of A-site Sm and B-site Mn/Ni co-doping on optical band gaps and the magnetic properties of BFO is studied in detail.Meanwhile,the SmFe1-xNixO3(SFNO)ceramics are synthesized by introducing Ni element into SmFeO3(SFO)ceramics.Its macrostructure and optical properties are investigated.X-ray diffraction(XRD)data show the BSF,BSFM and BSFMN ceramics and thin films have rhombohedral polycrystalline perovskite structures.From the surface morphologies,the ceramics show anomaly grain growth caused by the liquid phase formed in the sintering process.Dense and smooth microstructures are observed in the thin films.The lattice strains(?s)are estimated by Williamson-Hall(W-H)analysis model.The increase of strain values leads to more lattice distortions of the thin films.Increasing dopant content will decrease the band gaps of ceramics and thin films.For Mn doping,gap states are derived from the Mn 3d orbitals hybridized with the surrounding O 2p.For Ni doping,the O 2p state rises caused by the interaction between Ni and O.With the introduction of Mn and Ni,the saturation magnetizations increase,which could be observed in the magnetization hysteresis loops.Ions-doping inducing lattice distortions and thus suppressing cycloid spin structure,which results in ferromagnetism in the thin films.Rare-earth orthoferrites SFO ceramics have a large sinteing temperature zone.The diffuse reflectance spectra show that the optical band gap can be estimated to 2.06 eV.The charge disproportionation of Ni is the reason for SFNO band gap narrowing.2.The preparation,phase transition,physical properties of Pb(Zr0.53Ti0.47)O3-Pb(Fe0.5M0.5)O3(M = Nb/Ta,PZTFN/PZTFT)ceramics have been studied.According to the crystal structure research,the narrow band gap and ferroelectric properties of PZTFN/PZTFT materials are characterized and analyzed.The ferromagnetic orders are also observed in this series materials.These results suggest that PZTFN/PZTFT show potential application values of solar energy conversion and magneto-electric storage devices.PZTFN/PZTFT ceramics are prepared by solid-state reaction technique.The Pb(Fe0.5Nb0.5)O3(PFN)and Pb(Fe0.5Ta0.5)O3 are introduced into Pb(Zr0.53Ti0.47)O3 lattice.All the samples show polycrystalline perovskite structures with dense microstructures,and exhibit a gradual transition behavior from tetragonal to cubic structure.Raman spectra analysis for local symmetry shows local pseudocubic structures in high-doping samples.The PZTFN/PZTFT ceramics present narrowed band gaps,the Eg of PFN is 2.08 eV.Based on results of first principle calculation for PZT,the Eg narrowing mechanism is caused by the addition of Fe ions to increase tailing of band edge into the gap,which reduces the optical band gap of PZT.Furthermore,the Eg of PZTFT is slightly smaller than that of PZTFN with the same composition,which is due to the influence of grain size.The 0.1PZTFN possesses spontaneous polarization with good ferroelectric property.Meanwhile,it also shows ferromagnetism at room temperature for x= 0.1 component,and transforms to paramagnetism as x increases.The exchange interaction between local spin polarized electrons of Fe and conduction electrons is the main causes of ferromagnetism.However,the magnetization hysteresis loops of PZTFT show the paramagnetism with a linear curve,which implies that non-magnetic ions Nb5+ and Ta5+ play a role in the interaction.The different degrees of lattice distortion and strain induce the different magnetic properties of PZTFN and PZTFT.3.KNbO3-Ba(Fe0.5Nb0.5)O3(KBFNO)lead-free composite ceramics are synthesized by solid state reaction method.The structural phase transition,tunable narrow band gap,ferroelectric and ferromagnetic properties are studied in this class of materials.Furthermore,the lattice structure of BTO is modified by Bi(Zn0.5Ti0.5)O3(BZT)with a low tolerance factor.The influence of lattice structure on optical properties of Bi(Zn0.5Ti0.5)O3-BaTiO3(BBZTO)is investigated.XRD and Raman spectroscopy analysis suggest that KBFNO shows structural phase transition behavior from orthorhombic Amm2 to cubic Pm3m.The Bragg diffraction peaks of the KBFNO ceramics are indexed as polycrystalline perovskite structures.Raman spectra show the phenomenon of local component fluctuation and local cubic symmetry reduction.The continuous change of grain morphology from lamellar to irregular sphere structures is observed by scanning electron microscope.The wide band gap of KNO can be reduced by introducing BFNO.The lowest Eg is 1.15 eV for KBFNO ceramics.The nonlinear relation between band gaps and components x is discovered in KBFNO series materials,following the formula Eg = 3.21-5.65x + 4.17x2,which is called optical bowing effect.The band gap narrowing is mainly caused by the structural relaxation and charge exchange as ions doping.KBFNO ceramics show spontaneous polarization,which is due to Nb5+ shift off-center of NbO6 octahedron with spatial inversion symmetry breaking.As can be seen from the magnetization hysteresis loops,KBFNO shows a magnetic transition from diamagnetism to ferromagnetism when BFNO is introduced into KNO.It could be explained by the bound magnetic polaron theory.BTO is doped by BZT with low tolerance factor to prepare the BBZTO ceramics,which show tetragonal-to-rhombohedral phase transition.The morphotropic phase boundary is around x = 0.06.The lattice strain is estimated by the W-H analysis model,which suggests that the incorporation of substitution ions into the host lattice produces the inner stress field giving rise to structure distortions.BZT doping has little effect on band gap of BTO.The reduction of band gap is mainly due to the splitting of Zn 3d orbital caused by the electrostatic repulsion in tetragonal octahedral fields.With the increase of doping content,the phase transition leads to band gap broadened.For ABO3-A(B'B")O3 type lead-free materials,KBFNO is expected to break the limit of ferroelectric photovoltaic efficiency and has great potential in the application of absorber layer materials for solar cell.4.BFNO epitaxial thin films are successfully desposited on SrTiO3(STO)single crystal substrates.The technological parameters are optimized for preparing the BFNO thin films.First principles calculation and experimental results show a narrow band gap of BFNO.Meanwhile,the ferroelectric,ferromagnetic order and high dielectric constant properties are observed.The first-principles calculation results show that the band gap of BFNO is 1.6 eV.The electron transfer is a transition from occupied O 2p to unoccupied Fe 3d state,and a d-d transition between conduction band and valence band composed of Fe 3d.The BFNO single crystal thin film prepared under the optimum condition shows a smooth and dense surface.It is epitaxial grown along the c axis of STO substrate surface.The X-ray photoelectron spectroscopy suggests the existing of oxygen vacancies in the thin films.With the increase of substrate temperature,the oxygen vacancy defect reduces since the crystallinity promotion.The oxygen vacancy acts as a medium of two adj acent Fe3+ ions to form bound magnetic polaron,inducing macro ferromagnetism.Therefore,the sample desposited on 650 0C substrate temperature shows stronger ferromagnetism than that of 700 0C.A sandwich structure Au/BFNO/Nb:STO device is prepared using Au as the top electrode and Nb:STO as the bottom electrode.The ferroelectric and dielectric properties of BFNO are characterized.Ferroelectric hysteresis loops indicate favorable ferroelectric properties of BFNO thin films.In addition,BFNO thin film shows large dielectric constant,which decreases with the frequency increasing.