Lattice Vibrations And Electronic Transitions Properties Of Lead-free Ferroelectric Ceramics

Lead-free ferroelectric ceramic materials, owing to its excellent electrical properties and environmental friendliness, are one of the current hot researches of ferroelectric ceramic materials. At present, the lead-free ferroelectric research focuses on:1) the perovskite titanate ferroelectric materials, such as:barium titanate BaTiO3(BT), strontium titanate SrTiO3(ST), barium strontium titanate (BaxSr1-x) TiO3(BST), etc.2) type of tungsten bronze niobate ferroelectrics, such as strontium barium niobate SrxBa1-xNb2O6(SBN), etc.3) containing Aurivillius layer structure of ferroelectrics. Such as bismuth titanate Bi4Ti3O12, niobium acid strontium bismuth SrBi2Nb2O9(SBNO), etc. BST, SBN and SBNO and doping ferroelectric material with high resistivity, good fatigue resistance characteristic and high dielectric constant, pyroelectric detector, uv detector, uncooled infrared detectors, uncooled infrared focal plane arrays and ferroelectric memory, and other fields has great application prospect. With all walks of life to high heat release performance material such as electricity, high dielectric tunability of growing demand, type lead-free ferroelectric ceramic preparation, properties and modification studies has become a research hotspot of ferroelectric ceramic materials. According to the current scientific research workers of ferroelectric materials used for the analysis of the working principle of pyroelectric detector, the ideal working temperature scope of the pyroelectric detector should be in pyroelectric material near the Curie temperature (Tc), enhanced detector that is corresponding to the ambient temperature requirements. So to reduce this kind of phase change temperature point of ferroelectric material is as the next generation of pyroelectric detector development necessary conditions. Therefore, this paper systemly studies the type lead-free ferroelectric oxides and rare earth element doped ferroelectric materials such as Ba0.4Sr0.6-xMnxTiO3(BSMT), SrxBa1-xNb2O6(SBN) and SrBi2Nb2-xNdxO9(SBNN) and so on synthesis of ceramic, electronic structure and optical phonon vibration modes and the improvement of the Curie temperature characteristic using spectrometry measurements. In this paper, the main work and innovation points include the following:Ⅰ. The bismuth layer structure ferroelectric ceramics SBNN were studied in the temperature range of80-873K using Raman spectrum, infrared spectrum, and the ellipsometry spectroscopy. Detailed discussion in electronic band structure, optical phonon modes, and Curie temperature affected by the neodymium Nd doping in SBNN ceramics.The studies on SrBi2Nb2-xNdxO9(0≤x≤0.20) ceramics preparated by the solid state sintering reaction have shown that (a) Raman spectra analysis of SBNN at80K, The A1g[Nb] phonon mode located at207cm-1, origined from the octahedral NbO6oxygen, whose frequency increased with the increase of Nd components decreasing. With the increase of temperature, the peak position and strength of the A1g[Nb] phonon modes shows discontinuity in SBNN ceramics near the phase transition temperature.(b) The experimental spectra in the wavenumbers range of350-1500cm-1were analyzed using the Lorentz oscillator model for five infrared-active phononmode observed. It is found that the frequencies of the NbO6tilting and symmetric stretching modes linearly decrease with the Nd composition due to the octahedra distortion. The change of infrared-active phonon mode is consistent with the Raman-active behavior. The high frequency dielectric constant varies in the range from4.55to4.80, owing to the contribution from the stronger electronic transitions,(c) Base on the analysis on dielectric functions from0to500℃with Tauc-Lorentz dispersion model, the two interband transitions (located at3.70and4.78eV) have shown an abrupt variation near the phase transition region. The changes of dielectric functions are mainly due to the thermal-optical and/or photoelastic effect. Moreover, the characteristic alteration in interband transitions can be ascribed to distortion of NbO6octahedron and variation of hybridization between Bi6s and O2p states during the structure transformation. Finally, we found that the introduction of the rare ions in the Bi2O2layers can induce the tilting angle of NbO6octahedron obtained from Nb-O1-Nb bond angle, which decreases remarkably from9.7°for to5.5°. The phase transition temperature of SBNN ceramics decreases from about710to550K with increasing Nd composition, which provides a scientific basis for the development of the next generation of pyroelectric infrared detector materials.II. The dielectric functions of the perovskite ferroelectric BSMT ceramics in the photon energy range from ultraviolet to far-infrared have been investigated by the spectroscopy ellipsometry and infrared reflectance spectra. The lattice vibrations and electric band structure BSMT ceramics with different Mn doping composition are systematically studied.The Ba0.4Sr0.6-xMnxTiO3(BSMT) ceramics with different Mn composition (from1%to10%) have been prepared via the conventional solid-state reaction sintering,(a) The X-ray diffraction analysis shows that the ceramics are polycrystalline with the single perovskite phase.(b) The three first-order Raman-active phonon modes can be observed, and the frequency of the A1(LO3)/E(LO) mode shows a blue shift of8cm-1with the Mn composition, which can be attributed to the distortion of the TiO6octahedron. With increasing Mn composition, the frequency of the infrared-active TO4mode decreases from532to520cm-1, owing to the local variation of the lattice constant induced by the Mn incorporation,(c) The82increases with increasing Mn composition below5%, then sharply decreases at the Mn composition of10%in the photon energy range of1.0to3.0eV. It is found that the optical bandgap for the BSMT ceramics is varied between3.40and3.65eV with different Mn composition. A sharp increase of the Eg is observed when the Mn composition is10%. The present results could be crucial for future applications of ferroelectric-based optoelectronic devices. III. The Raman and infrared-active phonon modes in the low frequency range for the tungsten bronze ferroelectric SBN ceramics have been studies by temperature dependent Raman spectroscopy and infrared reflectance spectra. Form the soften and disppared of the A1g and soft mode, the Curie temperature of the SBN ceramics can be obtained as a function of Sr compositions.The lattice vibrational spectra of ferroelectric SrxBa1-xNb2O6(0.30≤x≤0.50) ceramics obtained by Ramna and infrared spectra can be affected by the variation of Sr/Ba ratios.(a) The frequency of IR-active T2u phonon mode decreases with increasing Sr composition, which can be ascribed to stronger deformation of the NbO6octahedra.(b) From the Raman analysis from150to750K, with increasing the temperature, the frequency of the A1g phonon mode decreases, the reduced intensity of the mode decreases as the temperature is increased. At the same time, the peak broadens with temperature. In general, a sharp change in the temperature dependence of both frequency and halfwidth of different Raman peaks was observed to occur at the phase transition. The soft modes of the SBN ceramics are located at about42cm-1. With increasing the temperature, the soft mode becomes soften, overdamped and disappeared. According to changes of the soft modes, it is clear that increasing Sr composition (from0.30to0.50) leads to the shrinking of T c (from450to375K) for phase transition:Tc (x)=556-371x.