钙钛矿铁电材料的晶格振动和相变规律研究

发布时间：2018-04-06 21:01

本文选题：钙钛矿结构　切入点：椭圆偏振光谱　出处：《华东师范大学》2015年博士论文

【摘要】：铁电氧化物是一类重要的信息功能材料体系,被广泛应用在数据存储和光电探测等领域,其中钙钛矿铁电材料,特别是多铁铁酸铋薄膜在室温下同时具有铁电性和反铁磁性,具有良好的磁电耦合、高极化率和光伏效应。此外,铁电陶瓷[PbZZr1-xTixO3(PZT), PbSc1/2Ta1/2O3(PST)等]作为新型的功能材料具有介电常数高、介电损耗低,压电和热释电性能良好等优点,使得它在高密度信息存储,高速处理,光电子技术以及通讯军事领域有着广阔的应用前景,而这些相关功能器件的研发跟对该特征材料体系的认识有密切的关系。通过调节脉冲激光沉积中不同的生长条件,改变衬底,调整Zr/Ti组分和掺杂可以进一步研究其内部复杂的物理特性,比如相变,大部分的相变物理机制是基于x射线衍射微观结构表征和介电频谱,电子能带结构和电子跃迁等信息是缺失的。因此铁电材料的物理性质研究在信息科学领域备受关注。光谱学手段是可用于材料表征的非破坏性探测技术,通过光谱分析,可以获得材料的光学常数,晶格振动,电子能带结构以及电子跃迁等重要信息。当材料处于变温条件下,可以了解声子行为和带间跃迁能量在相变中的演化规律,研究材料的相变物理机制,为研究相变材料的物理特性开辟崭新而有效的途径。本文的主要工作和创新点包括以下几点：1.研究了脉冲激光沉积法中激光功率,氧分压和薄膜压应力对铁酸铋薄膜晶格振动,光电跃迁,介电函数的影响。BFO声子频率随激光功率的增加而略微增大,禁带宽度随着氧分压的降低而减小。通过变温拉曼光谱和紫外-近红外透射光谱,发现A1(TO)声子频率和带隙随着温度的升高而变小。采用脉冲激光方法在c轴蓝宝石衬底上制备了不同激光功率下的BiFeO3薄膜。随着激光功率的增加,晶格常数c/a值变小,Bi/Fe组分比降低。位于219,172,142 cm-1的A1横向声子模式的频率随着温度的升高向低波数段移动,这是因为热膨胀,热无序和晶格的非谐效应。在200,500和640 K,低频Raman声子频率出现异常,这分别对应于磁相变,铁电序不同态间的变化和G-型反铁磁相变。激光功率造成Bi的缺失,Bi-O键长度和强度的变化以及Fe06八面体局部结构的畸变,使得E(TO)和A1(TO)声子频率略微增大。通过洛伦兹多振子模型拟合红外反射光谱得到50-8000cm-1范围内的介电函数,透明区介电函数随激光功率的变化与堆积密度,表面粗糙度和缺陷态有关。在蓝宝石衬底上不同氧分压条件下制备了BFO薄膜,随着氧分压的增加,拉曼活性E(TO1)声子频率从74增为76 cm-1,与薄膜中的拉应力有关。Fe2+离子和氧空位的浓度均随着氧分压的降低而增加。用Tauc-Lorentz色散模型拟合0.47-6.5 eV透射光谱得到介电函数。吸收边随着温度的升高(8-300 K)呈现红移趋势。禁带宽度Eg随着温度的升高而降低,与BFO薄膜中原子间距离影响的电子与声子相互作用有关。在8K时,随着氧分压的减小,Eg从2.88降为2.78 eV,这是由氧空位的增加在导带和价带之间形成的杂质态引起的。通过在衬底LaAl03上沉积不同厚度的BFO薄膜来调节薄膜中的压应力。薄膜均是高度(111)择优取向的,随着厚度的增加,压应变减小,薄膜与块状晶体之间的应变比计算为1.75,1.57和1。压应变促使Eg由2.7减小为2.65 eV,电荷转移跃迁能量由3.5增为4.1 eV。这与O2p态的移动和局部Fe3+晶体场的变化有关。2.研究了(1-x)PST-xPbHfO3(0≤x≤0.2)弛豫陶瓷的介电函数随温度的变化关系。观察到三个带间跃迁,且能量随温度的升高而降低。其中最低能量跃迁的线性温度系数随PbHfO3组分的增加先升高后降低。用变温椭圆偏振光谱研究不同组分PbHfO3陶瓷的电子能带结构和介电函数。参数ε2max在居里温度附近有最小值,这是因为在该温度下结构发生变化,从立方(顺电)到菱方相(铁电)的改变。根据临界点模型(SCP),从介电函数的二阶导可以得到三个典型的带间跃迁能量。跃迁能量对B位有序度较敏感,随着温度的升高呈现红移趋势,这与电子与声子的相互作用和晶格热膨胀有关。由于PbHfD3的加入,跃迁能量的线性温度系数受氧空位,B位原子(Sc,Ta,Hf)排布和Pb-O键的影响而变化。3.采用红外反射光谱和拉曼光谱研究Pb0.99(Zr0.95Ti0.05)0.98Nb0.0203-x wt.% MgO陶瓷的晶格振动在不同温度下的演变规律,MgO加入使得高温铁电菱方相到低温相的变化以及有序-无序转变温度降低。用远红外反射光谱(5.5-300 K)和拉曼光谱(77-300 K)来研究分析Mg掺杂的Pb0.99(Zr0.95Ti0.05)0.98Nb0.0203(PZTN)的晶格振动和相变。通过以上互补的实验手段,在室温下,所有的陶瓷均是低温铁电菱方相[FR(LT)]结构。采用洛伦兹多振子模型得到远红外区域的介电函数。与Pb-BO3(B=Zr,Ti,Nb)振动相关的低频声子模式主要影响远红外介电响应。随着MgO组分的增加,室温下的介电函数ε(0)分别为85.4,73.4,73.9,41.9。这是由Mg替代B位引起的双重电离氧空位引起的。通过声子频率,阻尼参数和强度随温度的演变规律,可以观察到120 K出的有序-无序相变。该温度随着MgO组分的增加略微减小,这与相关链断裂和局部固有偶极子产生引起的畸变有关。250 K附近观察到反铁电正交相Ao到FR(LT)的相变,这与110方向Pb原子的反铁电位移和角互连氧八面体的旋转有关。对于含MgO的PZTN陶瓷,在290 K附近有从低温菱方相到高温铁电菱方相[FR(HT)]的变化。这个变化与111方向阳离子(Pb和Zr/Ti/Nb/Mg离子)的偏移有关,此温度比纯的PZTN陶瓷的相变温度略小。
[Abstract]:The ferroelectric oxide is a kind of important functional material information system, is widely used in the field of data storage and photoelectric detection, the Perovskite Ferroelectric Materials, particularly iron bismuth ferrite thin films and ferroelectric antiferromagnetic at room temperature, with good magnetoelectric coupling, high polarization and the photovoltaic effect. In addition, ferroelectric ceramics [PbZZr1-xTixO3 (PZT), PbSc1/2Ta1/2O3 (PST), as a novel functional materials with high dielectric constant, low dielectric loss, piezoelectric and pyroelectric performance, making it high-speed processing in high density information storage, electronic technology and communication, the military field has a wide application prospect. And there is a close relationship between the development of these related functional devices with the characteristics of material system understanding. By adjusting the pulse laser deposition in different growth conditions, changing the substrate, adjust the Zr /Ti component and doping Further study on the physical characteristics of the internal complex such as phase change, phase change is most of the physical mechanism of X ray diffraction characterization of microstructure and dielectric spectrum based on the electronic band structure and electronic transition information is missing. So the research on the physical properties of ferroelectric materials have attracted much attention in the field of information science. Spectroscopy can be used for non destruction detection technology of materials characterization, by spectral analysis, can obtain the optical constants of materials, lattice vibration, electronic energy band structure and electron transition of important information. When the material is in variable temperature conditions, evolution can understand the phonon behavior and interband transition energy during the phase change, phase change material physical mechanism research, development a new and effective way to study physical properties of phase change materials. The main work and innovation points of this paper include the following: 1. study on the pulsed laser deposition method Laser power, oxygen partial pressure and film stress of BiFeO3 thin film lattice vibration, optical transition, frequency effect of.BFO phonon dielectric function with the increase of laser power and increases slightly, the band gap decreases with the decrease of oxygen pressure. The temperature dependent Raman spectra and UV Vis NIR transmission spectroscopy. A1 (TO) phonon frequency and band gap with the increases of temperature. The c axis sapphire substrate BiFeO3 films were prepared at different laser power by pulsed laser method. With the increase of laser power, the lattice constant c/a decreases, Bi/Fe component ratio decreased. At 219172142 cm-1 lateral A1 phonon mode frequency with increasing temperature to lower wavenumber mobile, this is because of thermal expansion, thermal disorder and lattice anharmonicity. In 200500 and 640 K, abnormal low frequency Raman phonon frequency, which correspond to the magnetic phase transition, electric iron sequence The changes between homomorphism and G- type antiferromagnetic phase transition. The laser power caused by the loss of Bi, distortion Bi-O bond length and strength as well as the Fe06 eight face of the local structure, the E (TO) and A1 (TO) increased slightly. The phonon frequency dielectric function in the range of 50-8000cm-1 by Lorenz oscillator model fitting infrared reflectance spectroscopy, transparent dielectric function area changes with the laser power and bulk density, surface roughness and defects. On sapphire substrates at different oxygen partial pressure under the conditions of the preparation of BFO films with the increase of oxygen partial pressure E (TO1), Raman active phonon frequency increased from 74 to 76 cm-1, and the film tensile force on the concentration of.Fe2+ ions and oxygen vacancies increased with decreasing oxygen pressure. The dielectric function obtained by using Tauc-Lorentz dispersion model fitting 0.47-6.5 eV spectra. The absorption edge with increasing temperature (8-300 K) showed red shift Eg. The band gap decreased with the increase of temperature, and the electron phonon effect of distance between atoms in BFO film is related to the interaction. In the 8K, with the decrease of oxygen partial pressure, Eg decreased from 2.88 to 2.78 eV, which is increased by the oxygen vacancy impurity formed between the valence band and conduction band caused by adjust the pressure in the film. The deposition of BFO thin films with different thickness on the substrate of LaAl03 to stress. Films are highly (111) preferred orientation, with the increase of the thickness, the compressive strain decreases, the strain between the film and the bulk crystal ratio were 1.75,1.57 and 1. strain Eg decreased from 2.7 to 2.65 eV and the transition energy increased from 3.5 to 4.1 eV. and the O2p state of the mobile Fe3+ and local crystal field changes related to the charge transfer of.2. (1-x) PST-xPbHfO3 (x = 0 ~ 0.2) dielectric function of relaxor ceramics with temperature change. The relationship between observed three interband transitions, and can The amount decreases with the increase of temperature. The increase of linear temperature coefficient of the lowest energy transition with PbHfO3 component increased first and then decreased. Studied by spectroscopic ellipsometry at different electronic components PbHfO3 ceramic band structure and dielectric function parameters. 2max has a minimum value near the Curie temperature, this is because in the temperature structure changes from cubic to rhombohedral (PE) (ferroelectric) change. According to the critical point model (SCP), from the two order derivative of the dielectric function can be obtained by three typical interband transition energy. The transition energy of the ordered degree of B is sensitive, with the temperature increasing. With this trend, the electron phonon interaction and lattice thermal expansion. Due to the addition of PbHfD3, the linear temperature coefficient of transition energy by oxygen vacancies, B atoms (Sc, Ta, Hf) and Pb-O bond arrangement effect and the changes of.3. by infrared reflectance spectroscopy and Raman Spectra of Pb0.99 (Zr0.95Ti0.05) evolution at different temperatures 0.98Nb0.0203-x wt.% lattice vibration of MgO ceramics, the addition of MgO makes high temperature ferroelectric rhombohedral phase and low temperature to change the order disorder transition temperature decreased. With the far infrared reflection spectra (5.5-300 K) and Raman spectroscopy (77-300 K) to study the Mg doped Pb0.99 (Zr0.95Ti0.05) 0.98Nb0.0203 (PZTN) of the lattice vibration and phase transition. By means of experiment above complementary, at room temperature, all ceramic are low temperature ferroelectric rhombohedral structure. [FR (LT)] by Lorenz oscillator model to obtain the dielectric function of far infrared region. (B=Zr, Ti, and Pb-BO3 Nb) low frequency phonon modes mainly affect the vibration of far infrared dielectric response. With the increase of MgO concentration, the dielectric function epsilon at room temperature (0) were replaced by Mg B 85.4,73.4,73.9,41.9. which is caused by the double ionized oxygen vacancies Cause. The phonon frequency, damping parameters and strength with the evolution of temperature, can be observed that the order disorder phase transition 120 K. The temperature with the increase of MgO concentration slightly decreased, which is associated with the chain fracture and local dipoles caused by the distortion of the.250 K near the observed antiferroelectric orthorhombic phase Ao FR (LT) phase rotation on anti iron electric displacement and angle of interconnection in eight face and 110 oxygen atoms of the Pb direction. For PZTN ceramics containing MgO, around 290 K from low temperature to high temperature rhombohedral ferroelectric rhombohedral [FR (HT)] change. This change with the 111 direction cation (Pb and Zr/Ti/Nb/Mg ions) offset, the temperature of phase transition temperature of PZTN ceramics is smaller than pure.

【学位授予单位】：华东师范大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TB34;TQ174.1

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