【作者】 杨新建；

【导师】 李世春； 雷毅；

【作者基本信息】 中国石油大学 ， 化工材料， 2009， 博士

【摘要】 钙钛矿型铁电材料是一类应用十分广泛的功能材料。铁电性的有效利用和新功能的发现都有赖于理论上对铁电性内在本质的深入理解,铁电性的本质起源问题至今尚未完全解决。铁电现象是原子层次的效应,对晶体的对称性十分敏感,其物理机理是晶胞中原子的坐标发生了微小变化,导致对称破缺。低的晶体对称性使物理本构关系矩阵复杂化,因此其他理论(如密度泛函理论,分子动力学理论)描述起来更加困难。这正好是价键理论(经验电子理论)可以使用的切入点。本文采用价键电子理论——固体与分子经验电子理论(EET),较全面地跟踪研究了典型的钙钛矿型铁电材料,从晶体结构出发,计算了它们的价键电子结构,对价键电子结构随晶体结构、温度和组份的变化给予相应的理论描述。并以价键电子结构为基础,讨论了钙钛矿型铁电材料的铁电性、压电性等物理性质,从而沟通了材料微观价键电子结构与某些宏观性质之间的联系。按晶体结构从简单到复杂、研究历史由过去到现在的顺序,选取五种典型的、具有较大理论研究价值和实际应用价值或者较大的预期应用价值的钙钛矿铁电材料(简单钙钛矿型铁电材料、无铅铁电材料、固溶体钙钛矿型铁电材料、新型弛豫铁电单晶和单相多铁性材料)分别进行理论研究。计算结果不但从价键理论角度再现了钙钛矿型氧化物化学键离子性与共价性相混合的特点,而且说明了铁电性主要源于过渡金属离子与氧离子共价性化学键沿自发极化方向出现的非均匀分布。计算分析了无铅钙钛矿型铁电材料与钛酸铅价键电子结构之间的差异,从价键电子结构角度分析了无铅铁电材料性能差距的根源。针对钙钛矿型固溶体铁电材料晶体结构的复杂性和特殊性,构造了带权重的平均原子模型计算方法,计算分析了不同B位原子对铁电性的贡献。对新型弛豫铁电晶体准同型相界四方相一侧的价键电子结构进行计算,明确了MPB附近四方相一侧的价键电子结构随着组分和温度的变化情况。以多铁性材料铁酸铋为研究对象,从价键电子结构的计算结果讨论了铁电性和铁磁性共存的可能性。更多还原

【Abstract】 The perovskite ferroelectrics are important functional materials. Theoretical researches are not only helpful to understand the phase transition mechanism of ferroelectrics, but also advantaged to the application of ferroelectric materials. However, the origin of ferroelectric behavior is not very clear nowadays.The ferroelectric distortions involve small displacements of the cations relative to the anions, leading to a net dipole moment per unit volume, i.e. a polarization. Lower lattice symmetry makes the description more difficult for conventional methods such as density functional theory and molecular dynamics simulations. But it is convenient for the valence bond theory to deal with low lattice symmetry problems.In this dissertation, the valence electronic structures of five kinds of typical perovskite ferroelectrics are investigated according to the valence bond theory, the empirical electron theory (EET) of solids and molecules.In simple perovskite ferroelectrics, the bond of atom B and atom O is the strongest bond, which is responsible for the tendency to the ferroelectricity. It is found that the valence electronic structure is different between cubic phase and tetragonal phase and the B-O bond changes into two kinds of different bonds along the tetragonal axis direction as the transition occurs.In lead-free ferroelectrics, the difference of the valence electronic structures between the lead titanant and lead-free materials is found. In tetragonal ferroelectric phase, the B-O bond is less weaker than that in PbTiO3. B-O bond is essential to the ferroelctricity of these materials. The spontaneous polarizations of KNbO3 and BaTiO3 are smaller than PbTiO3.In solid solution ferroelectrics, the B-site atoms make different contributions to the ferroelectricity. In PZT, atom Ti is more important than atom Zr. With the increase of Ti content, the B-O bond becomes stronger. In KTN, atom Ni is more important than atom Ta.In new single crystal relaxor ferroelectrics, valence electronic structures of the Morphotropic phase boundary (MPB) will change with the temperature and the composition of these materials.In single phase multiferroelectric, the possibility of the co-existence of the magnetism and ferroelectricity is found after the calculation of valence electronic structure. The Fe-O bond is not the shortest bond in the crystal but is the strongest co-valence bond, which is the origin of the ferreoelectricity and atom Fe has intrinsic magnetic moment from d electron.It is showed by EET that the bond B-O is very important to the ferroelctricity in these perovskite ferroelectrics.更多还原