几种典型材料负膨胀机理的第一性原理研究

发布时间：2018-05-26 15:34

本文选题：第一性原理计算 + 负热膨胀　；参考：《郑州大学》2016年博士论文

【摘要】：随着新技术、新方法、新手段的出现,负热膨胀材料作为材料科学领域的新宠,得到了越来越多的关注。由于目前已有的负热膨胀材料大多为绝缘体,虽有大量的工作采用掺杂、与金属复合等手段改善其导电性能,但金属性的缺失仍然极大地限制了其在电、热、机械等领域的应用。本论文着眼于负热膨胀材料的导电性能,采用基于密度泛函理论的第一性原理方法对几种负热膨胀材料的反常热膨胀机理进行研究,并与相关的绝缘负热膨胀材料对比,去解释负热膨胀特性与导电性的内在联系。本论文的研究内容和结论如下:1.实验报道了立方结构的Re O_3不仅具有负热膨胀特性,而且其优异的导电性可与Ag、Cu媲美。借助于第一性原理和准谐近似,我们的计算发现:倒空间M和R点处的格林艾森参数具有负向极大绝对值。与此对应的低频光学支的振动引起Re O_6刚性八面体耦合转动,导致体积受热收缩。O-2p和Re-t2g电子态的杂化使其呈现优良的导电特性。实验还指出与其同为立方结构的绝缘体Sc F3,呈现出更强烈的负热膨胀特性,负热膨胀系数约是Re O_3的十倍。计算发现二者负热膨胀成因一样,均为M和R点的格林艾森参数出现负极值,以及低频光学支引起的刚性八面体耦合转动。热胀系数差别巨大归因于电子结构的不同。Re-O共价键强于Sc-F离子键,而且按照M和R点振动模式移动原子,发现O原子移位势垒比F要陡。这均使得Re O6八面体的扭转变得困难,导致热胀系数偏小。2.立方相Re O_3可认为是A-位缺失的ABO_3型钙钛矿结构。A-位原子缺失所致的体心充足空位使得Re O_3很容易出现相变。随着压强增加,Re O_3呈现的一系列相变过程中,四方相P4/mbm是否存在,一直存在争议。在本章节中我们利用第一性原理计算,借助于声子谱振动曲线、形成热焓和动力学能量势垒,澄清了从立方Pm-3m到P4/mbm相变的可能性,并定量给出转变压强是5.0 kbar。虽然Re O_3有多种结构相,但只有立方Pm-3m相存在负热膨胀现象。通过计算不同压强值下的色散关系和对晶体结构分析,立方Pm-3m相在5.0 kbar时,倒空间M点的M3振动模出现虚频,结构不再稳定。M3振动模是立方相出现负热膨胀现象的原因,M3模的软化对应于O原子从x=0.2500移位至x=0.2401,出现相变,负热膨胀现象消失。3.过渡金属镧系碳化物,不仅具有良好的机械性能,还是潜在的超导材料。实验发现四方相La C2在低温区呈现超导电性,还表现出各向异性的负热膨胀特性。在本章节中,借助于第一性原理计算和准谐近似,我们模拟再现了跨越超导温区的负热膨胀现象。计算结论显示c-轴的负热膨胀系数和温区均与实验吻合一致。计算结果还预言了可能由于实验仪器精度不够而没能测出的a-轴的负热膨胀现象。通过计算格林艾森参数和因子群分析,布里渊区中心的Eu和Eg模,边界M和Z点的三支振动模对应负格林艾森参数值,C-C二聚体的横向振动引起晶格La……La有效距离缩短,导致负热膨胀现象。4.实验发现,随着温度升高,钙钛矿Bi Ni O_3从低温三斜相转变为高温正交相的过程中,出现了显著的负热膨胀现象,而且还伴随有绝缘体→导体和反铁磁→铁磁的转变。这使得Bi Ni O_3的负热膨胀现象貌似与磁、电、热振动均相关,使其负热膨胀机理更复杂。本章节从第一性原理出发,通过计算分析电、磁和晶体结构,证实:高温正交相中,由于Bi-6s与O-2p电子态的杂化,使得费米能级处出现杂化峰,表现为导电性,磁矩为1.732μB/Ni。通过分析电子态密度,发现了Ni与Bi金属间电荷转移。BVSs计算表明两个相的氧化态分别是Bi+3Ni+3O_3和Bi+30.5 Bi+50.5Ni+2O_3.电荷态密度图显示低温相为G-型反铁磁绝缘体,而且伴随着金属间电荷转移,体系从小体积正交相变成大体积三斜相,出现负热膨胀行为。我们的理论计算很好的解释和证实了金属间电荷转移是Bi Ni O_3呈现负热膨胀的成因,为丰富负热膨胀的机理解释提供了新内容。
[Abstract]:With the emergence of new technology, new methods and new means, negative thermal expansion materials have been paid more and more attention as the new favorite in the field of material science. Because most of the existing negative thermal expansion materials are insulators, although a large number of work is used to improve their electrical properties, such as doping and metal complex, the loss of metal is still great. Its application in electrical, thermal and mechanical fields is restricted. This paper focuses on the conductivity of negative thermal expansion materials. The first principle method based on density functional theory is used to study the anomalous thermal expansion mechanism of several negative thermal expansion materials and to compare with the related negative thermal expansion materials to explain the negative thermal expansion characteristics and the characteristics of negative thermal expansion. The contents and conclusions of this paper are as follows: 1. the experiment reported that the cubic structure of Re O_3 not only has negative thermal expansion, but its excellent conductivity is comparable to that of Ag and Cu. With the help of the first principle and quasi harmonic approximation, our calculation shows that the Green Eisen parameter at the inverted space M and R points has a negative pole. Large absolute value. The vibration of the corresponding low frequency optical branch caused by the coupling rotation of the Re O_6 rigid eight body, resulting in the hybrids of the.O-2p and Re-t2g electronic states of the volume heating shrinkage to show excellent conductivity. The experiment also pointed out that the Sc F3 of the insulator with the same cubic structure showed a more intense negative thermal expansion characteristic and the negative thermal expansion coefficient. It is about ten times that of Re O_3. It is found that the negative thermal expansion of the two is the same as the Green Eisen parameter of M and R points, and the rigid eight body coupling rotation caused by the low frequency optical branch. The difference of thermal expansion coefficient is greatly attributed to the different.Re-O covalent bond of the electronic structure than the Sc-F ion bond, and the vibration mode of the M and R points is moved. It is found that the O atom shift barrier is steeper than that of F, which makes the torsion of the Re O6 eight surface difficult, resulting in a small.2. cubic phase Re O_3, which is considered to be the ABO_3 type of the ABO_3 type perovskite structure of the A- bit loss of the.A- bit atom, which makes the Re O_3 easily appear in the phase transition. In a series of phase transitions, there is always controversy whether the Quartet phase P4/mbm exists. In this chapter, we use the first principle to calculate the enthalpy and kinetic energy barrier by means of the phonon spectrum vibration curve, clarify the possibility of the phase transition from cubic Pm-3m to P4/mbm, and give a quantitative transition pressure of 5 kbar. although Re O_3 is much more. But only cubic Pm-3m phase has negative thermal expansion. By calculating the dispersion relation and crystal structure analysis under different pressure values, the M3 vibration mode of the M point in the inverted space appears imaginary frequency at 5 kbar, and the structure is no longer stable the.M3 vibration mode is the cause of the negative thermal expansion of the cubic phase, and the softening of the M3 mode corresponds to the.M3. O atoms shift from x=0.2500 to x=0.2401 and appear phase transition. Negative thermal expansion disappears.3. transition metal lanthanide carbides, not only with good mechanical properties, but also potential superconducting materials. The experiment found that the tetragonal phase La C2 presents superconductivity at low temperature, and shows the negative thermal expansion of anisotropy. In this chapter, with the help of the first part We simulate and reproduce the negative thermal expansion of the superconducting zone. The calculation results show that the negative thermal expansion coefficient and the temperature zone of the c- axis are in agreement with the experiment. The results also predict the negative thermal expansion of the a- axis, which may not be measured by the precision of the experimental instrument. The Eu and Eg modes of the Brillouin center, the Eu and Eg modes of the Brillouin center and the three vibrational modes at the boundary M and Z point to the negative Green Eisen parameter value. The transverse vibration of the C-C two polymer causes the effective distance of the lattice La.La to be shortened, and the negative thermal expansion is caused by the.4. experiment. The perovskite Bi Ni O_3 changes from the low temperature three oblique phase to the high temperature as the temperature rises. In the process of intersection, there is a significant negative thermal expansion, and also with the transformation of insulators, conductors and antiferromagnetism to ferromagnetic. This makes the negative thermal expansion of Bi Ni O_3 appear to be related to magnetic, electric and thermal vibration, and the mechanism of its negative thermal expansion is more complex. From the first principle, the analysis of electricity, Ci Hejing The body structure shows that, in the orthogonal phase of high temperature, the hybrid peak of the Fermi energy is caused by the hybridization of the electronic state of Bi-6s and O-2p. The magnetic moment is 1.732 u B/Ni. by analyzing the electron state density. The calculation of the charge transfer.BVSs between the Ni and Bi metals shows that the oxidation states of the two phases are Bi+3Ni+3O_3 and Bi+30.5 Bi+50.5Ni+2O_3., respectively. The charge density map shows that the low temperature phase is G- antiferromagnetic insulator, and with the charge transfer between metals, the system becomes a large volume three diagonal phase from small volume quadrature phase to negative thermal expansion. Our theoretical calculation explains and confirms that the charge transfer between metals is the cause of negative thermal expansion of Bi Ni O_3, which is rich in negative heat. The explanation of the mechanism of expansion provides new content.
【学位授予单位】：郑州大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O469;O551.3

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