钛酸铅基化合物晶体结构及其负热膨胀性Word文件下载.doc

1、中 文 摘 要钛酸铅（PbTiO3）是一种重要的钙钛矿结构的铁电体，在介电、压电、铁电、热释电等方面具有重要的研究与应用价值；同时，它在室温至居里温度范围内还表现出奇特的热缩冷胀行为，即负热膨胀性（NTE），这种负热膨胀行为是其它钙钛矿结构化合物所不具有的，如CaTiO3、BaTiO3、KNbO3、BiFeO3等。研究PbTiO3的负热膨胀性将有利于开发出负热膨胀性可控以及零膨胀材料，拓展负热膨胀材料在实际中的应用，PbTiO3负热膨胀机理的研究可指导新型负热膨胀材料的开发。本论文主要以钙钛矿结构的铁电材料Pb1-xAxTi1-yByO3（ALa、Sr、Cd、Bi、（La1/2K1/2）等；B

2、Fe、Zn等不同价态金属原子）为中心，研究A位与B位替代对其负热膨胀性、晶体结构、点阵动力学的影响，实现负热膨胀性能可控，开发零膨胀材料，并研究PbTiO3负热膨胀机理。本文研究了Pb1-xAxTiO3（ALa、Sr、（La1/2K1/2）、Cd）体系的固溶体特性、晶体结构以及负热膨胀性能受掺杂的影响。La、Sr、（La1/2K1/2）的掺杂都使PbTiO3的轴比（c/a）及居里温度（TC）不同程度地线性下降，La的掺杂大幅度地降低了PbTiO3的负热膨胀性能，在0.15 xLa 0.20范围内，Pb1-xLaxTiO3表现出零膨胀性能。PbTiO3-CdTiO3体系中，Cd的A位替代不仅提高

3、了PbTiO3的轴比（c/a），而且增强了其负热膨胀效应，这是目前所发现的唯一能使PbTiO3负热膨胀性得到增强的A位掺杂体系。Pb1-xLaxTiO3晶体结构研究发现，La的掺杂使四方相点阵晶格中Pb/La与Ti原子的自发极化位移（c轴方向）非线性降低，然而位移比值（dPb/La/dTi）呈线性降低趋势，氧八面体具有刚性特征不受La掺杂的影响；Pb1-xCdxTiO3体系X射线与中子衍射联合晶体结构研究发现，Pb1-xCdxTiO3体现反常的晶体结构特征，虽然轴比（c/a）增加，但是Pb/Cd与Ti原子自发极化位移反常下降，从而导致居里温度的略微降低。在目前已知的A与B位复合替代PbTiO3

4、基化合物当中，（1-x）PbTiO3-xBiFeO3固溶体中，BiFeO3的加入使PbTiO3的晶体结构反常变化，轴比、居里温度以及负热膨胀性能同时增加，当BiFeO3含量为x0.6时，在很宽的温度范围内（25650）具有强烈的负热膨胀性，平均体热膨胀系数为-3.9210-5-1，高于Sleight在1998年报道的具有最强负热膨胀性能的氧化物AlPO4-17（-3.5010-5-1）。中子衍射全谱拟合研究发现，当x0.3时，Ti与Fe并不随机占据同一位置（1b），而是在c轴方向上发生劈裂，Ti更靠近氧八面体中心；然而随着BiFeO3含量的增加到接近准同型相边界x0.6时，Ti与Fe随机分布于

5、同一位置。价键模型计算的各原子价态进一步支持了该晶体结构的合理性。（1-x）PbTiO3-xBiFeO3体系具有高的自发极化，在压电、铁电等方面具有潜在的应用价值。本文成功地在单一相PbTiO3-（Bi,La）FeO3化合物中实现了负热膨胀性能控制，平均体热膨胀系数范围-0.52-4.6510-5-1，已经覆盖了目前已报道的所有具有负热膨胀性能的氧化物范围，并且该化合物具有负热膨胀性温度范围宽、合成容易、高温稳定等特点。在零膨胀材料设计方面，本文研究出一种温度范围宽（-140700）的零膨胀均相材料（PbTiO3-0.1BiFeO3-0.3Bi（Zn1/2Ti1/2）O3），发现局部原子的无序

6、分布是产生低膨胀以及实现零膨胀的重要手段，为今后的零膨胀材料设计提供了一种有效途径。Raman光谱晶格动力学以及晶体结构精细修正研究表明，纵光学模与横光学模（LO-TO）的劈裂程度以及Pb-O2键距与负热膨胀行为具有高度的一致变化特性，表明Pb-O电子轨道杂化不仅是PbTiO3铁电性的起源，而且对负热膨胀性起着重要的作用，为目前研究负热膨胀材料提出一种新的机理。关键词：负热膨胀性，晶体结构，钛酸铅基化合物，Rietveld方法，点阵动力学Structure and Negative Thermal Expansion in the Lead Titanate-Based CompoundsJu

7、n ChenABSTRACTLead titanate （PbTiO3） is one of important perovskite-type ferroelectrics which has been widely investigated and applied in the field of dielectric, piezoelectricity, ferroelectricity and pyroelectricity. Furthermore, PbTiO3 behaves a novel thermal property, that is negative thermal ex

8、pansion （NTE）, whose unit cell volume contracts on heating in the temperature range of room temperature to the Curie point （TC）. The NTE property of PbTiO3 is very different with almost all other numbers of perovskites, such as CaTiO3, BaTiO3, KNbO3 and BiFeO3. The investigations on the NTE of PbTiO

9、3 will be helpful for the control of NTE, design of zero thermal expansion （ZTE） materials and exploitation of the NTE materials application. Additionally, the NTE mechanism of PbTiO3 would be applied for the design of new NTE materials. In the present dissertation, the perovskite-type ferroelectric

10、s Pb1-xAxTi1-yByO3 （A = La, Sr, Cd, Bi, （La1/2K1/2）; B = Fe, Zn） were selected to investigate the A and B-site substitution dependence of NTE, crystal structure and lattice dynamics, so as to realize the control of NTE, exploit ZTE materials, and study the mechanism of NTE of PbTiO3.In the Pb1-xAxTi

11、O3 （A = La, Sr, （La1/2K1/2） and Cd） compounds, the properties of solubility, crystal structure and NTE were investigated. The substitution of Pb by La, Sr and （La1/2K1/2） reduces the tetrogonality （c/a） and the Curie temperature linearly. The NTE is reduced dramatically by La doping in PbTiO3, and P

12、b1-xLaxTiO3 compounds exhibit ZTE in the solubility range of 0.15 xLa 0.20. For the PbTiO3-CdTiO3, the substitution of Pb by Cd not only increases the c/a ratio but also enhances the NTE, which is only found that can raise both the tetragonality and the NTE in the A-site doped PbTiO3 up to now. As f

13、or the crystal structures of Pb1-3/2xLaxTiO3, the atom displacements of spontaneous polarization （Ps） for Ti （dTi） and Pb/La （dPb/La） along the c-axis direction decrease nonlinearly with increasing La content, while the value of dPb/La/dTi decreases linearly. The calculated value of PS decreases lin

14、early in the solubility range of 0.05 x 0.30. The shape of oxygen octahedron TiO6 is independent of the La content. Pb1-xCdxTiO3 exhibits an abnormal crystal structure by the means of the x-ray and neutron diffraction. Although the tetragonality （c/a） is enhanced, the PS displacements of Pb/Cd and T

15、i slightly decrease resulting in the decrease in the Curie temperature.In the known A and B-site combined substitutions of PbTiO3-based compounds, the structure of PbTiO3 is abnormally affected by the BiFeO3 substitution, where the tetragonality （c/a）, the Curie temperature （TC） and the NTE are all

16、enhanced. As the x = 0.6, the compound of 0.4PbTiO3-0.6BiFeO3 behaves a very strong NTE with an average bulk thermal expansion coefficient （TEC） of = -3.9210-5-1 which is larger than that of AlPO4-17 （ = -3.5010-5-1） once reported by Sleight as the strongest NTE material in 1998. The structures of （

17、1-x）PbTiO3-xBiFeO3 （x = 0.3 and 0.6） were investigated by the neutron powder diffraction. A splitting shift between Fe and Ti atoms was found along the c axis in 0.7PbTiO3-0.3BiFeO3; however, this splitting disappears in the 0.4PbTiO3-0.6BiFeO3 near the morphotropic phase boundary （MPB）. The reliabi

18、lity of the refinement results was also supported by the bond-valence calculations （BVC）. The tetragonal phase of PbTiO3-BiFeO3 exhibits a large spontaneous polarization with a potential application in the ferroelectrics and piezoelectrics.The NTE is successfully controlled on a large scale in the c

19、ompounds of PbTiO3-（Bi,La）FeO3 with an average TEC of = -0.52-4.6510-5-1 which includes that of all previously reported NTE oxides. PbTiO3-（Bi,La）FeO3 possesses the merits of the wide temperature range of NTE, facile synthesis and high-temperature stabilization. As for design of the zero thermal exp

20、ansion, the compound of 0.6PbTiO3-0.1BiFeO3-0.3Bi（Zn1/2Ti1/2）O3 exhibits the ZTE in a wide temperature range of -140700.A mechanism is suggested that the low thermal expansion and ZTE could be realized by the introduction of local disorder of atoms, which could be an effective method for the design

21、of ZTE materials.The investigations of Raman scattering lattice dynamics, crystal structures and NTE properties reveal that the splitting of between longitudinal optical （LO） and transverse optical （TO） phonons and bonding distance of Pb-O2 are highly related to NTE, which suggests that the Pb-O hyb

22、ridization not only is the origin of ferroelecity, but also plays an important role in the NTE. Pb-O hybridization might be the nature of NTE for PbTiO3, which is a new kind of mechanism for NTE materials.Key words: Negative thermal expansion, Crystal structure, PbTiO3-based compound, Rietveld method, Lattice dynamics