若干铁电体的负热膨胀机理及调控研究

发布时间：2018-03-20 19:47

本文选题：负热膨胀　切入点：铁电体　出处：《北京科技大学》2017年博士论文　论文类型：学位论文

【摘要】：铁电体(Ferroelectrie)作为重要的固体功能材料,具有铁电、压电、介电和热释电等性能,在现代电子工业中应用广泛。同时因其晶格具有反常的负热膨胀性(negative thermal expansion,NTE)而受到关注。从二十世纪初,本课题组重点研究了钛酸铅(PbTi03)及其化合物中的NTE,取得了诸多成果,提出了自发体积铁电致收缩(spontaneous volume ferroelectrostriction,SVFS)的概念。在对钛酸铅基铁电体的电子结构、晶体结构、自发极化(Ps)及NTE的大量研究后,认为Pb6s-O 2p杂化作用不仅是晶体结构畸变的关键,也是引发其NTE的核心因素。第一性原理计算表明Pb-O键的热振动对钛酸铅的NTE有重要贡献。基于上述结果,本文进一步研究铁电体NTE中的基本问题。首先探究了格林艾森定律(Gruneisen relation)在钛酸铅NTE中的运用。虽然PbTiO3的主要光学支包括铁电软模具有显著的负格林艾森系数,但并不能描述钛酸铅的NTE特征。对PbTiO3软模的非谐性分析显示其本征非谐性为重要部分,因此由准谐近似推导的格式定律在此出现异常结果。对比NTE增强的钛酸铅基化合物,发现软模的本征非谐性与其NTE大小呈正相关。利用氧同位素18O对PbTiO3晶格动力学的改变,仔细研究了热振动在其NTE中的作用。室温高分辨同步辐射的衍射分析发现18O减小了 PbTiO3的晶格常数a,而使沿Ps方向的晶格常数c反常增大(各约万分之一)。18O对晶格各向异性的改变与PbTiO3的各向异性热膨胀行为一致。结合变温Raman和PbL3的EXAFS研究证实了 Pb-O热振动在极化轴热收缩中的作用,进一步认识了钛酸铅NTE行为的基础。本文在无铅铁电体中确定并研究了硫属族化合物Sn2P2S6的NTE(243～338 K,一4.7×10-5/K)。结合晶体结构分析与第一性原理计算,由Sn5s-S3p杂化解释了其NTE过程。Sn2P2S6的Ps相对PbTi03明显偏弱,但仍具有显著NTE行为,因此推测这类阴阳离子的s-p杂化作用是铁电体NTE实现的必要条件。利用同主族的、具备与S3p不同杂化能力的Pb2+/Ge2+进行对Sn2+的替代,以研究其对NTE的调控。最后选取有Sb 5s-S 3p杂化作用的SbSI铁电体作了进一步验证,确定了较为明显的NTE行为(253～293 K,一5.0×10-5/K),支持了上述推测。另外在BaTiO3中的A位引入Ca2+后虽大幅增强了极化轴的热收缩、削弱约60%的体积热膨胀系数,但仍未得到可观的NTE,从侧面验证了 s-p杂化作为铁电体NTE的必要条件。本文总结得到了具有NTE行为铁电体的基本特点:s-p杂化和铁电软模(位移型相变)。铁电体的软模具有显著的本征非谐性,升高温度带来相关原子非谐热振动的增强,削弱和破坏s-p杂化,导致Ps的减小以及极化轴的剧烈热收缩,实现单胞体积的NTE。而在不具备上述条件的铁电体中则很难实现NTE。以上认识给铁电体NTE的设计和调控提供了重要思路。
[Abstract]:As important solid functional materials, ferroelectrics have ferroelectric, piezoelectric, dielectric and pyroelectric properties. It has been widely used in modern electronics industry and has attracted much attention because of its anomalous negative thermal expansion. From the beginning of 20th century, our group has focused on the study of lead titanate (PbTi03) and its compounds, and has achieved many results. In this paper, the concept of spontaneous bulk ferroelectric shrinkage volume ferro electrostriction (SVFS) is proposed. After a large number of studies on the electronic structure, crystal structure, spontaneous polarization Pss and NTE of lead titanate ferroelectrics, it is concluded that the hybrid action of Pb6s-O 2p is not only the key to the distortion of crystal structure. First principle calculation shows that the thermal vibration of Pb-O bond contributes significantly to the NTE of lead titanate. In this paper, the basic problems in ferroelectrics NTE are further studied. Firstly, the application of Gruneisen relation in lead titanate NTE is investigated. Although the main optical branches of PbTiO3 include the ferroelectric soft mode, it has a significant negative Greeniesen coefficient. But it can not describe the NTE characteristics of lead titanate. The anharmonic analysis of PbTiO3 soft mode shows that the intrinsic anharmonic property is an important part, so the format law derived from quasi harmonic approximation has abnormal results. Compared with the NTE enhanced lead titanate compounds, It is found that the intrinsic anharmonic property of the soft mode is positively correlated with its NTE size. The lattice dynamics of PbTiO3 is changed by using the oxygen isotope 18O. The effect of thermal vibration on NTE is carefully studied. The diffraction analysis of high resolution synchrotron radiation at room temperature shows that 18O decreases the lattice constant a of PbTiO3 and increases the lattice constant c in the direction of Ps (about 1/10000? 18O each). The change of lattice anisotropy is consistent with the anisotropic thermal expansion behavior of PbTiO3. The effect of Pb-O thermal vibration on the thermal shrinkage of the polarized axis is confirmed by combining the EXAFS study with variable temperature Raman and PbL3. The basis of the behavior of lead titanate (NTE) is further understood. In this paper, the NTE(243~338 K, -4.7 脳 10 ~ (-5) / K ~ (-1) of the sulfur compound Sn2P2S6 is determined and studied in lead-free ferroelectrics. The crystal structure analysis is combined with the first-principle calculation. The PS of NTE process. Sn2P2S6 is obviously weaker than that of PbTi03, but it still has significant NTE behavior. Therefore, it is inferred that s-p hybridization of this kind of anion is a necessary condition for the realization of ferroelectric NTE. Pb2 / Ge2 with different heterozygosity from S3p was used to replace Sn2 in order to study its regulation on NTE. Finally, the ferroelectrics of SbSI with Sb5s-S3p hybrid were selected for further verification. The obvious NTE behavior of 253103K, -5.0 脳 10 ~ (-5) / K ~ (-1), supports the above conjecture. In addition, the introduction of Ca2 in the A position in BaTiO3 greatly increases the thermal shrinkage of the polarization axis and weakens the volumetric thermal expansion coefficient of about 60%. The necessary conditions for s-p hybrid as ferroelectric NTE have been verified from the side. In this paper, the basic characteristics of ferroelectrics with NTE behavior:: s-p hybrid and ferroelectric soft mode (displacement-type phase transition) are summarized. Soft modules have remarkable intrinsic anharmonic properties. The increase of temperature leads to the enhancement of anharmonic thermal vibration of related atoms, the weakening and destruction of s-p hybridization, and the decrease of Ps and the sharp thermal contraction of the polarization axis. It is difficult to realize NTE in ferroelectrics without the above conditions. The above understanding provides an important idea for the design and regulation of ferroelectrics NTE.
【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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