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

Ferroic oxides are important functional materials with stable structure,excellent performance and wide application.They have flexible and diverse crystal structures and elemental composition,which makes them display outstanding doping tolerance.It can not only achieve the regulation of structural symmetry,energy band distribution and spin configuration through ion-doping in specific lattice position,but also can be combined with other materials as solid solution or composites to make multiple degrees of freedom coupling via elastic interaction,so as to induce the multifunctionality of the materials.Through different ways of performance optimization,ferroic oxides show a broader application prospect in low-power and high-density logic memory,spintronic devices,energy collectors and other multifunctional devices.In addition,ferroelectric oxides have attracted much attention because of their unique bulk photovoltaic effect.However,their wide application in photovoltaic field is still limited by the intrinsic wide band gap.Therefore,the exploration of ferroic oxides with narrow band gap and its application in solar photovoltaic devices has become an important target of the research of ferroic materials.In this paper,Fe-based ferroic oxides are the main line of research,focusing on Fe-based perovskite oxides,Fe-based double perovskite oxides and spinel ferrites.BiFeO₃（BFO）-based,（K,Na）_0.5Bi_0.5TiO₃（（K,N）BT）-based and KNbO₃（KNO）-based ferroic materials were prepared by solid-state reaction method,sol-gel technique and Pulsed Laser Deposition technique.The experimental strategies such as ion-doping,solid solution and composite were used to optimize the performance of the materials,and the crystal structure,micromorphology,energy band structure and optical,electrical and magnetic properties were deeply studied by different characterization and testing methods,providing new ideas for the research and development of multifunctional materials and devices.The innovative achievements of this paper are summarized as the following points:1.The optical band gap and spin magnetic order of BFO are optimized by ion-doping.Bi_0.7A_0.3FeO₃（BAFO,A=Ca,Sr,Pb,Ba）ceramics doped with bivalent ions at Bi-site have been prepared by solid state reaction method.La³⁺and Mn²⁺doped polycrystalline thin films Bi_0.85La_0.15FeO₃（BLFO）,BiFe_0.85Mn_0.15O₃（BFMO）and Bi_0.85La_0.15Fe_0.85Mn_0.15O₃（BLFMO）have been prepared by sol-gel method,in order to explore the influence of ion-doping on the structure and properties of BFO ceramics and films.X-ray diffraction spectroscopy（XRD）and Raman scattering spectroscopy show that the BAFO ceramics sintered at high temperature and BFO-based films are both polycrystalline perovskite structures.Both ceramic and film samples undergo the phase transition from rhombohedral phase to cubic phase caused by ion-doping.Besides,lattice expansion and contraction phenomenon have been observed in both ceramic and thin film samples due to the difference of the doped ion radius.The Williamson-Hall analysis method has been used to estimate the lattice strain of the films,and the lattice distortion caused by ion-doping was verified.Raman scattering spectroscopy shows that ion-doping at the Bi-site and Fe-site causes the change of Raman modes in different bands,further confirming that ion-doping can affect the vibration of Bi-O bond,Fe-O bond and FeO₆ in different ways.Scanning electron microscopy（SEM）results show that,compared with the BFO sample,the grains of BAFO change from ellipse to square,the grain sizes are significantly reduced,and the boundary is clearer.La³⁺doping helps to improve the density of the films,while Mn²⁺ doping leads to the increase of oxygen vacancies in the crystals,and the growth quality of BLFO and BLFMO films is better than that of BFMO films.The optical band gaps（Eg）of pure BFO ceramics and thin films were 1.97 eV and 2.56 eV,respectively.The Eg of BCFO ceramics decreased to 1.96 eV,while the Eg of BSFO,BPFO and BBFO ceramics increased compared with that of BFO.This phenomenon was caused by smaller chemical compression induced by lattice expansion.Mn²⁺ doping can introduce the band-tail state and effectively narrow the band gap of BFO thin films,while La³⁺ doping can widen the band gap due to the Burstein-Moss effect,which reduces the Eg of BLFMO thin films to 2.36 eV.The hysteresis loop（M-H）recorded at room temperature shows that doping of Sr²⁺,Pb²⁺ and Ba²⁺ can cause the transition from antiferromagnetic to ferromagnetic in ceramic samples,which is due to the lattice distortion caused by ion-doping,thus destroying the intrinsic cycloid spin structure of BFO.2.Band gap narrowing,magnetic and electric properties optimizing of BFO-based ceramics have been further realized by forming solid solution and composites.（1-x）Bi_0.7Ba_0.3Fe_0.7Coo.iTi_0.2O3-xBaFe_0.35Co_0.15Nb_0.5O₃（（1-x）BBFCT-xBFCNO）solid solution and（1-x）BiFeO₃-xZnFe₂O₄（（1-x）BFO-xZFO）composite ceramics have been prepared by solid state reaction method.The two methods both can adjust the structure and properties of BFO-based oxides effectively,especially showing outstanding performance in band gap narrowing.XRD analysis indicated that the BFO-based solid solution and composite ceramics both show good crystallinity.Perovskite BBFCT and double perovskite BFCNO have cubic structures,while their solid solutions transform into rhombohedral structures,revealing the existence of lattice distortion.Two sets of independent diffraction peaks have been found in the XRD patterns of the（1-x）BFO-xZFO composite ceramics,indicating that the two phases exist independently in the composite ceramics and no chemical reaction occurs.In（1-x）BBFCT-xBFCNO solid solution ceramics,Raman modes of the three bands are obviously changed,indicating that the addition of BFCNO has an effect on Bi-O bond,Fe-O bond and FeO₆ octahedral in BBFCT.In（1-x）BFO-xZFO composite ceramics,the changes of Raman modes in the medium and high frequency ranges are more obvious than those in the low frequency range,indicating that ZFO mainly acts on Fe-O bond and FeO₆ octahedral.The optical band gaps of 0.4BFCT-0.6BFCNO and 0.4BFO-0.6ZFO ceramics were reduced to 1.56 eV and 1.83 eV,respectively,by solution and recombination.The optical band gaps of 0.4BBFCT-0.6BFCNO and 0.4BFO-0.6ZFO ceramics were reduced to 1.56 eV and 1.83 eV.The band gap narrowing is mainly due to the changing of FeO₆ crystal field induced by the lattice distortion.Ferromagnetism and ferroelectricity have been found in（1-x）BBFCT-xBFCNO solid solution ceramics.The variation of the magnetic properties can be explained by the Dzyaloshinski-Moriya（DM）interaction between different ions and the bound magnetic polarity theory.（1-x）BFO-xZFO composite ceramics exhibit a linear M-H relationship,which is due to paramagnetic ZFO in normal spinel structure at room temperature.Therefore,the intrinsic cycloid spin structure of BFO cannot be effectively improved.3.Study on the structure and properties of the composites forming with perovskite oxide（K,Na）_0.5Bi_0.5TiO₃（（K,N）BT）and spinel ferrites.（1-x）K_0.5Bi_0.5TiO₃-xCoFe₂O₄（（1-x））KBT-xCFO)and K_0.25Na_0.25Bi_0.5TiO₃（KNBT）-based composite ceramics have been prepared by solid state reaction method.The experiments show that the composite of ferroelectric phase and ferromagnetic phase is an effective way to realize multiferroic materials.Meanwhile,spinel ferrites can effectively narrow the intrinsic wide band gap of KBT and KNBT perovskite oxides,making them become more ideal ferroelectric photovoltaic materials.XRD patterns showed that two groups of independent XRD peaks were detected in（1-x）KBT-xCFO and KNBT-based composite ceramics,which characterized the perovskite oxide phase and spinel ferrite phase,respectively.Changing of Raman modes for the two groups of composite ceramics have been discussed from low frequency,intermediate frequency and high frequency ranges.The addition of spinel ferrites mainly changed the middle-frequency Raman modes.It is because of the ion exchanging between spinel phase and KBT/KNBT phase,influencing the bond length and angle of Ti-O bond.In KBT-based composite ceramics,the increase of CFO phase can effectively improve the absorption ability of KBT in the visible light range,thus making the band gap of the composite ceramics decreasing from 3.320 eV to 1.331 eV.The addition of spinel phase Co_0.8Ni_0.2Fe₂O₄（CNFO）,Co_0.8Zn_0.2Fe₂O₄（CZFO）and Co_0.8Ni_0.1Zn_0.1Fe₂O₄（CNZFO）can reduce the band gaps from 3.38 eV of KNBT to 2.50 eV and 2.63 eV and 2.58 eV,respectively.M-H loops show that the addition of CFO phase can make KBT change from antiferromagnetism to ferromagnetism,and the saturation magnetization increases with the increase of CFO content.The ferromagnetism mainly comes from the superexchange between metal ions in the tetrahedral and octahedral sublattices of CFO phase.P-E loops indicate that spinel ferrites also play a prominent role in optimizing the electrical properties of the composite ceramics.Among them,the addition of spinel phase NCFO,ZCFO and NZCFO significantly enhances the ferroelectric properties of KNBT-based composite ceramics,and all electrical parameters are significantly improved.4.The optimizing of the optical,electrical and magnetic properties of KNbO3（KNO）-based ceramics by Fe-based double perovskite oxides has been investigated.Firstly,the Fe-based double perovskite Ba(Fe_1/3Ni_1/3Nb_1/3)O₃（BFNNO）epitaxial single crystal thin film was prepared by Pulsed Laser Deposition technique.（1-x）KNbO₃-xBa(Fe_1/3Ni_1/3Nb_1/3)O₃（KBFNNO）and（1-x）KNbO₃-xBa(Co_1/3Ni_1/3Nb_1/3)O₃（KBCNNO）solid solution ceramics were prepared by solid state reaction method.The results prove that Fe-based double perovskite oxides can effectively optimize the properties of KNO-based materials,especially on the band gap narrowing.The BFNNO single crystal films were prepared by Pulsed Laser Deposition technique,and the synthesis process of the films has been explored and optimized.The experimental results show that 700℃ is the best substrate temperature for the synthesizing BFNNO single crystal films.Thickness of the thin films prepared at 650℃was calculated by X-ray reflectometry（XRR）pattern,which turns out be around 61.538 nm,closing to the thickness measured by SEM cross section.The optical properties of BFNNO single crystal films were analyzed by Photoluminescence（PL）spectra.The results display that the optical band gap of the films is about 0.99 eV.In KBFNNO solid solution,the multiple characteristic peaks of KNO merge into a single peak,which confirms the phase transition from orthorhombic to cubic.At the same time,the diffraction peak is shifted to low angles,which reveals the increase of interfacial spacing and the expansion of cell volume.The change of Raman modes further indicates the effect of BFNNO on the crystal structure and symmetry of KNO.The Fe-based double perovskite oxide BFNNO greatly improves the band gap of KNO,making the solid solution exhibit a narrow band gap of 1.62 eV.At the same time,the addition of BFNNO makes KNO transform from intrinsic diamagnetism to ferromagnetism,and enhances the polarization to some extent.Finally,KBCNNO solid solution ceramics and KBFNNO material systems have been comparatively studied.The results show that the Fe-based double perovskite BFNNO can narrow the optical band gap of KNO more effectively.In terms of magnetism,BCNNO ceramics are ferromagnetic at room temperature,but the solid solution of BCNNO and KNO show paramagnetism,which proves that BCNNO is inferior to BFNNO in terms of magnetic optimization.As for electrical properties,the solid solution of KBCNNO show a nearly linear P-E relationship,which indicates that BCNNO could not improve the ferroelectric properties of KNO.The three experiments j ointly proved the superiority of performance of Fe-based double perovskite oxide BFNNO.