典型钡基铁电及弛豫铁电体电子显微学及第一性原理研究
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摘要
本论文采用电子显微术和第一性原理计算方法，研究了典型铁电体BaTiO_3和弛豫铁电体Ba(Ti_(1-x)Sn_x)O_3的微观结构与材料铁电和介电性能之间的关系。
采用分子动力学方法和Girshberg-Yacoby模型研究了BaTiO_3的顺电-铁电相变过程。计算结果表明BaTiO_3顺电-铁电相变过程具有明显的有序-无序型铁电相变特征，铁电软模频率与温度之间满足Cochran线性关系。Girshberg-Yacoby模型揭示了BaTiO_3相变过程中介电函数发散现象的物理本质，它起源于软模与BaTiO_3中Ti离子的有序-无序之间的耦合。
利用暗场成像和像差校正高分辨成像方法分别研究了成分为16%，20%和25%的Ba(Ti_(1-x)Sn_x)O_3的显微结构。暗场成像方法和像差校正高分辨图像的定量分析结果均表明成分为20%和25%的弛豫铁电体BTSn中存在着静态位移极化微区结构；而在成分为16%的铁电体BTSn中则只观察到具有较小位移的结构，不具有明显的极化微区结构。该实验结果表明弛豫铁电体中纳米尺度静态极化微区的产生可能是材料出现弛豫铁电性的重要因素。
采用电子能量损失谱与第一性原理相结合的方法，分别研究了成分为16%，20%和25%的Ba(Ti_(1-x)Sn_x)O_3的能量损失函数。第一性原理计算得到的能量损失函数定性上与实验结果吻合。计算结果表明Ti-O成键具有强烈的轨道杂化，有助于铁电极化的稳定；Sn-O成键的轨道杂化效应较弱，因此Sn-O的铁电极化被抑制。该结果有助于理解BTSn材料从铁电性到弛豫铁电性转变的物理机制。
利用第一性原理方法系统研究了BaMO3(M=Ti、Zr、Hf和Sn)的价带电子结构，Born有效电荷和原子相互作用力常数。计算结果表明Born有效电荷与轨道杂化之间存在着密切的联系，轨道杂化越强则Born有效电荷越大。对原子间相互作用力的分析表明，铁电极化的稳定与原子间的长程偶极-偶极相互作用力相关，而长程偶极-偶极相互作用力主要取决于原子的Born有效电荷，该结果成功地将材料的电子结构与物理性质统一为一个整体。
The microstructure of typical ferroelectric BaTiO_3and relaxor ferroelectricBa(Ti_(1-x)Sn_x)O_3were investigated to understand their ferroelectric and dielectric proper-ties using electron microscopy and first-principles calculations.
The dynamic structure, the eigenfrequency of ferroelectric soft mode as a functionof temperature and dielectric divergence in the paraelectric-ferroelectric phase transitionof BaTiO_3were studied by effective Hamiltonian molecular dynamics and Girshberg-Yacoby model. The calculation results showed that the phase transition of BaTiO_3is oforder-disorder type and the eigenfrequency of soft mode as a function of temperatureobeys the Cochran linear law. Using the Girshberg-Yacoby model, it was revealed thatthe physical mechanism of dielectric divergence in the phase transition originates fromthe coupling between soft mode and the order-disorder Ti ions. As a result, the dielec-tric function is strongly dependent upon temperature and characterized by a divergentbehavior.
The microstructure of Ba(Ti_(1-x)Sn_x)O_3(x=16%,20%and25%) was studied usingdark-field imaging and aberration-corrected high-resolution transmission electron mi-croscopy. The characterized structural diffuse scatterings along {100} were used fordark-field imaging and we found that there are characterized by bright static spot struc-tures in the relaxor BTSn (x=20%and25%), while they were not observed in the ferro-electric BTSn (x=16%). The atomic-resolution structures of BTSn were further inves-tigated and quantitative analysis of the HRTEM images showed that there exist staticpolar nanoregions in relaxor BTSn(x=20%and25%) with size of2nm. In ferroelec-tric BTSn(x=16%), only structures with small displacements were found. The resultsindicated that the static polar nanoregions may be a key factor of the materials’ relaxorproperties.
The energy-loss fucntions (ELF) of BTSn were studied by combining electronenergy-loss spectroscopy and first-principle calculations. The theoretically predictedELFs are qualitatively in agreement with the experimental results. All the peaks in thedielectric functions and ELFs are explained based on the calculated electronic structuresbelow the Fermi surface. It is found that the Ti-O bond is strongly hybridized, whichhelps to stabilize the ferroelectric distortions. As a comparison, the ferroelectric instable Sn-O is suppressed due to the weak hybridization of Sn-O bond.
The valence electronic structures, Born effective charges (BEC) as well as inter-atomic force constants (IFC) of BaMO3(M=Ti,Zr,Hf and Sn) were systematically stud-ied by first-principles calculation. The spatial distribution of O2p valence electrons ofthesecompoundsare obtainedintheframeworkof Wannierfunctions. Wefoundthatthed orbitals of the transition metals Ti, Zr and Hf are all hybridized with O2p orbitals andthe3d orbitals of titanium are the most delocalized, while those of Zr and Hf are morelocalized. There are no evident hybridizations between tin and O2p orbitals. Furtheranalysis of the BECs revealed that the BECs are intimately associated with orbital hy-bridizations. Analysis of the IFCs indicate that the BECs and long-range dipole-dipoleinteractions between atoms play important roles in the promotion of ferroelectric insta-bilities.
采用分子动力学方法和Girshberg-Yacoby模型研究了BaTiO_3的顺电-铁电相变过程。计算结果表明BaTiO_3顺电-铁电相变过程具有明显的有序-无序型铁电相变特征，铁电软模频率与温度之间满足Cochran线性关系。Girshberg-Yacoby模型揭示了BaTiO_3相变过程中介电函数发散现象的物理本质，它起源于软模与BaTiO_3中Ti离子的有序-无序之间的耦合。
利用暗场成像和像差校正高分辨成像方法分别研究了成分为16%，20%和25%的Ba(Ti_(1-x)Sn_x)O_3的显微结构。暗场成像方法和像差校正高分辨图像的定量分析结果均表明成分为20%和25%的弛豫铁电体BTSn中存在着静态位移极化微区结构；而在成分为16%的铁电体BTSn中则只观察到具有较小位移的结构，不具有明显的极化微区结构。该实验结果表明弛豫铁电体中纳米尺度静态极化微区的产生可能是材料出现弛豫铁电性的重要因素。
采用电子能量损失谱与第一性原理相结合的方法，分别研究了成分为16%，20%和25%的Ba(Ti_(1-x)Sn_x)O_3的能量损失函数。第一性原理计算得到的能量损失函数定性上与实验结果吻合。计算结果表明Ti-O成键具有强烈的轨道杂化，有助于铁电极化的稳定；Sn-O成键的轨道杂化效应较弱，因此Sn-O的铁电极化被抑制。该结果有助于理解BTSn材料从铁电性到弛豫铁电性转变的物理机制。
利用第一性原理方法系统研究了BaMO3(M=Ti、Zr、Hf和Sn)的价带电子结构，Born有效电荷和原子相互作用力常数。计算结果表明Born有效电荷与轨道杂化之间存在着密切的联系，轨道杂化越强则Born有效电荷越大。对原子间相互作用力的分析表明，铁电极化的稳定与原子间的长程偶极-偶极相互作用力相关，而长程偶极-偶极相互作用力主要取决于原子的Born有效电荷，该结果成功地将材料的电子结构与物理性质统一为一个整体。
The microstructure of typical ferroelectric BaTiO_3and relaxor ferroelectricBa(Ti_(1-x)Sn_x)O_3were investigated to understand their ferroelectric and dielectric proper-ties using electron microscopy and first-principles calculations.
The dynamic structure, the eigenfrequency of ferroelectric soft mode as a functionof temperature and dielectric divergence in the paraelectric-ferroelectric phase transitionof BaTiO_3were studied by effective Hamiltonian molecular dynamics and Girshberg-Yacoby model. The calculation results showed that the phase transition of BaTiO_3is oforder-disorder type and the eigenfrequency of soft mode as a function of temperatureobeys the Cochran linear law. Using the Girshberg-Yacoby model, it was revealed thatthe physical mechanism of dielectric divergence in the phase transition originates fromthe coupling between soft mode and the order-disorder Ti ions. As a result, the dielec-tric function is strongly dependent upon temperature and characterized by a divergentbehavior.
The microstructure of Ba(Ti_(1-x)Sn_x)O_3(x=16%,20%and25%) was studied usingdark-field imaging and aberration-corrected high-resolution transmission electron mi-croscopy. The characterized structural diffuse scatterings along {100} were used fordark-field imaging and we found that there are characterized by bright static spot struc-tures in the relaxor BTSn (x=20%and25%), while they were not observed in the ferro-electric BTSn (x=16%). The atomic-resolution structures of BTSn were further inves-tigated and quantitative analysis of the HRTEM images showed that there exist staticpolar nanoregions in relaxor BTSn(x=20%and25%) with size of2nm. In ferroelec-tric BTSn(x=16%), only structures with small displacements were found. The resultsindicated that the static polar nanoregions may be a key factor of the materials’ relaxorproperties.
The energy-loss fucntions (ELF) of BTSn were studied by combining electronenergy-loss spectroscopy and first-principle calculations. The theoretically predictedELFs are qualitatively in agreement with the experimental results. All the peaks in thedielectric functions and ELFs are explained based on the calculated electronic structuresbelow the Fermi surface. It is found that the Ti-O bond is strongly hybridized, whichhelps to stabilize the ferroelectric distortions. As a comparison, the ferroelectric instable Sn-O is suppressed due to the weak hybridization of Sn-O bond.
The valence electronic structures, Born effective charges (BEC) as well as inter-atomic force constants (IFC) of BaMO3(M=Ti,Zr,Hf and Sn) were systematically stud-ied by first-principles calculation. The spatial distribution of O2p valence electrons ofthesecompoundsare obtainedintheframeworkof Wannierfunctions. Wefoundthatthed orbitals of the transition metals Ti, Zr and Hf are all hybridized with O2p orbitals andthe3d orbitals of titanium are the most delocalized, while those of Zr and Hf are morelocalized. There are no evident hybridizations between tin and O2p orbitals. Furtheranalysis of the BECs revealed that the BECs are intimately associated with orbital hy-bridizations. Analysis of the IFCs indicate that the BECs and long-range dipole-dipoleinteractions between atoms play important roles in the promotion of ferroelectric insta-bilities.
引文
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