铌酸铅基钨青铜化合物晶体结构与热膨胀性

发布时间：2018-01-04 09:31

本文关键词：铌酸铅基钨青铜化合物晶体结构与热膨胀性　出处：《北京科技大学》2017年博士论文　论文类型：学位论文

【摘要】：四方钨青铜结构化合物是除钙钛矿结构化合物之外最大的一类铁电体材料,在压电、介电等领域有着重要的研究价值和应用前景。四方钨青铜型铌酸铅基化合物由于具有居里温度高、宏观极性强等特殊物理性质和晶体结构的多样性而受到关注,对其热膨胀行为的研究和加深对钨青铜类化合物固体化学本质的认识有助于解决这类材料在实际应用当中遇到的热应力问题。本论文以热膨胀性为出发点,选择、设计并合成了一系列四方钨青铜型铌酸铅基化合物；系统地研究了(特殊)晶体结构和晶体结构的演变规律；测试其热膨胀性,并揭示了轴向正、负热膨胀机理及与晶体结构、电子结构之间的关系；设计出通过调控极性来调控热膨胀性的方法。发现了具有优异非线性光学性能的非化学计量比的四方钨青铜结构化合物,为极性材料的设计提供新思路。首先用变温X射线衍射方法确定了Pb2KNb5O15具有b方向的轴向负热膨胀性。变温中子衍射精细结构修正结果表明铁电相内Pb、Nb原子具有a-b面内的极化位移；b方向的负热膨胀是由八面体结构之间耦合旋转与畸变直接导致的。第一性原理计算表明具有6s2孤对电子的Pb2+与O2-形成Pb-O共价键导致了铁电极化的增强。Pb2KNb5O15的轴向负热膨胀具有框架结构和铁电热致收缩两重特性,而铁电热致收缩是其根本原因。通过引入不同离子,研究了Pb2RNb5O15(R=Na.K0.5Li0.5、K、Rb、Ag)以及Pb3TiNb4O15(PTN)中R离子对其热膨胀性的调控。实验结果表明,引入小半径R离子的化合物倾向于形成a-b面内和c方向的规律性超结构,并确定了R=K0.5Li0.5、Ag时的晶体结构。小半径R离子能够有效提高居里温度并增强铁电极性,进而增强极轴b方向上的负热膨胀性；同时,小半径R离子倾向于引起八面体的倾斜,进而使c方向正膨胀加剧。材料的宏观热膨胀由这两个对立因素平衡的结果所决定。采用具有6s2孤对电子的Bi3+取代(PbR)3+,设计并合成新化合物PbBiNb5O15。选区电子衍射实验观察到PbBiNb5O15样品中高达五阶的非公度调制卫星峰,表明其存在很强的非公度调制特性。结构解析发现PbBiNb5O15中A位Pb2+和Bi3+具有大的位置和占位调制幅度,进而形成大的局域偶极矩。PbBiNb5O15通过非公度的非周期性调制,使得a-b面内的局域偶极矩在长程上得以抵消,并导致了较弱的宏观极性。通过Li的引入得到具有优异非线性光学性能的非化学计量比四方钨青铜化合物Pb2.15Li0.7Nb5O15。结合中子衍射、X射线衍射精确解析出Pb2.15Li0.7Nb5O15晶体结构并确定了Li占据四边形孔洞位置,澄清了Li在钨青铜中具体位置的争议。原子级分辨率透射电镜成像表明,Pb、Li随机分布导致了局域极性增强,这可能是优异非线性光学性质的原因。
[Abstract]:The tetragonal tungsten bronze compounds are a class of ferroelectric materials except the largest perovskite compound, in piezoelectric, dielectric and other fields have important research value and application prospect. The tetragonal tungsten bronze type niobate lead compounds with high Curie temperature, extremely strong macro diversity and special physical properties of crystals the structure of attention, and to deepen the Study on the thermal expansion behavior of the solid understanding of the chemical nature of tungsten bronze compounds contribute to the solution of this kind of material encountered in the practical application of the thermal stress problem. This paper expands as a starting point, to heat the choice, design and a series of tetragonal tungsten bronze type niobium lead acid based compounds were synthesized; systematically studied (special) evolution of crystal structure and crystal structure; to test its thermal expansion, and reveals the mechanism of axial, and negative thermal expansion and crystal structure, electronic structure The relationship between the structure design; through the regulation of the polarity of the method. The non stoichiometric regulation of heat expansion with excellent nonlinear optical properties than the tetragonal tungsten bronze compounds were found, and provide new ideas for the design of polar material. First determine the axial direction of the B Pb2KNb5O15 with negative thermal expansion with temperature X ray diffraction method. Thermal neutron diffraction results show that the fine structure of modified ferroelectric phase Pb, Nb atoms with A-B in-plane displacement polarization; negative thermal expansion B direction is directly caused by the structure coupling between the eight rotation and distortion. The first principle calculation shows that 6s2 has a lone electron pair Pb2+ and O2- lead the enhanced axial negative thermal expansion of.Pb2KNb5O15 with frame structure and electric iron induced shrinkage characteristics of the ferroelectric polarization Pb-O covalent bond formation, and the electric iron induced shrinkage is the fundamental reason. Through the introduction of different Ion on Pb2RNb5O15 (R=Na.K0.5Li0.5, K, Rb, Ag) and Pb3TiNb4O15 (PTN) R ion on the thermal expansion of regulation. The experimental results show that the regularity of the introduction of small radius of R ions tend to form compounds in A-B plane and c direction of the super structure, and to determine the crystal structure of R= K0.5Li0.5. Ag. The small radius of R ions can effectively improve and enhance the Curie temperature of iron electrode, thus enhancing the negative thermal expansion of b axis direction; at the same time, the small radius of R ions tend to cause eight inclined surface body, and the c direction is inflation intensified. Macro thermal expansion materials determined by the two a factor balanced results. With the 6s2 lone pair electrons of Bi3+ substituted 3+ (PbR), the design and synthesis of new compounds PbBiNb5O15. electron diffraction observed incommensurate modulated satellite peaks in PbBiNb5O15 samples up to five order, show the presence of strong non The incommensurate modulation characteristics. Structure analysis found in PbBiNb5O15 A Pb2+ and Bi3+ with high position and occupying the modulation amplitude, and the formation of local dipole moments.PbBiNb5O15 through non periodic modulation of incommensurate, the local A-B plane dipole moment can be offset in the long term, and lead to the weaker polarity. With the introduction of Li are nonstoichiometric with excellent nonlinear optical properties than the tetragonal tungsten bronze compounds Pb2.15Li0.7Nb5O15. with neutron diffraction, X ray diffraction analytical Pb2.15Li0.7Nb5O15 crystal structure and determine the Li occupied the quadrilateral hole position, to clarify the specific location of the Li tungsten bronze in the dispute. Atomic resolution transmission electron microscopy imaging showed that Pb, Li, random distribution causes local polarity enhancement, which is probably the reason why the excellent nonlinear optical properties.

【学位授予单位】：北京科技大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TM221

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