无机功能材料电子—声子及声子—声子耦合的计算机模拟研究

发布时间：2019-02-11 19:39

【摘要】：晶格动力学是现代固体物理的基础之一。晶体中的原子在热激发下,不断地在平衡位置附近振动。这些由原子集体振动所产生的声子可以与许多激发态发生耦合,其中最主要的耦合是:电子-声子和声子-声子耦合。它们决定了材料中与电子和声子输运相关的许多物理性质,比如金属的电导率、超导电性和热导率等。本论文选取高压下氢化物和铁基方钴矿热电材料作为研究电子-声子和声子-声子耦合的对象,采用基于密度泛函理论的第一性原理从头算方法,进行了系统性的输运性质研究,获得如下创新性成果:1.高压下预测的两个富氢磷族化合物(AsH_8和SbH_4)的超导转变温度都超过了100K;发现了二元氢化物高压性质的一般化学趋势。系统探索了磷族氢化物的高压相图,发现所有的磷氢化物高压下都倾向于分解,砷氢和锑氢化物中发现存在两个稳定的富氢化合物(AsH_8和SbH_4)。AsH_8和SbH_4的超导转变温度(Tc)都超过100K。特别是SbH_4具有最高的能量稳定性,其合成压力只有150GPa。通过对已探索的二元氢化物的理论数据挖掘,我们发现了氢化物高压性质的一般化学趋势,其高压下的热力学稳定性、成键特征和电声耦合等性质与组成元素在常压下的电负性差存在紧密的联系。该研究工作为寻找稳定的固态氢化物以及探索高温超导电性提供了有价值的理论指导。2.发现了二元未填方钴矿材料FeSb_3具有超低的本征晶格热导率,改变了人们在方钴矿体系中对热输运规律的传统认识。室温下,FeSb_3的晶格热导率只有1.14W/m K,是同类材料CoSb_3的十分之一。填充原子并未导致FeSb_3的晶格热导率的降低,这改变了人们在方钴矿体系中的传统认识(填充原子会显著地降低方钴矿材料的晶格热导率)。FeSb_3中的超低晶格热导率主要来自于整个声子谱的软化,尤其是与结构中Sb-Sb共价键关联的低频光学支声子的软化相关。3.发现高电负性元素填充的方钴矿SnFe_4Sb_(12)具有超低的本征晶格热导率,为优化方钴矿材料的热电性能提供了新的途径。我们发现高电负性的Sn原子在填充进入FeSb_3的晶格空位时,有中心偏离现象,进而导致Sn在FeSb_3中具有“Goldstone”声子模式,这是SnFe_4Sb_(12)室温下具有超低本征晶格热导率(0.69W/m K)的主要原因。进一步地,我们设计了双填方案,将在SnFe_4Sb_(12)研究中得到的成果应用到实际的方钴矿热电性能的改善中,为制备高性能的方钴矿热电材料提供新的途径。
[Abstract]:Lattice dynamics is one of the foundations of modern solid physics. The atoms in the crystal vibrate in the vicinity of the equilibrium position under thermal excitation. These phonons produced by collective atomic vibration can be coupled with many excited states, the most important of which are electron-phonon and phonon-phonon coupling. They determine many physical properties related to electron and phonon transport, such as the conductivity, superconductivity and thermal conductivity of metals. In this paper, the hydride and ferroelectric materials at high pressure are selected as the objects of electron-phonon and phonon-phonon coupling, and the first-principle ab initio method based on density functional theory is used. A systematic study of the transport properties is carried out, and the following innovative results are obtained: 1. The superconducting transition temperatures of the two HP-rich compounds (AsH_8 and SbH_4) predicted at high pressure are over 100K, and the general chemical trend of the high pressure properties of binary hydride has been found. The high pressure phase diagrams of phosphorous hydride have been systematically explored, and it has been found that all of them tend to decompose at high pressure. Two stable hydrogen-rich compounds (AsH_8 and SbH_4) were found in arsenic and antimony hydride. The superconducting transition temperatures of AsH_8 and SbH_4 were both over 100K. In particular, SbH_4 has the highest energy stability, its synthesis pressure is only 150 GPA. By mining the theoretical data of the explored binary hydride, we find out the general chemical trend of the high pressure properties of the hydride and its thermodynamic stability under high pressure. The properties of bonding and electro-acoustic coupling are closely related to the electronegativity difference of the constituent elements at normal pressure. This work provides valuable theoretical guidance for finding stable solid hydride and exploring high temperature superconductivity. 2. It is found that the binary unfilled galactic cobalt (FeSb_3) material has ultra-low intrinsic lattice thermal conductivity, which changes the traditional understanding of thermal transport law in galactic cobalt system. At room temperature, the lattice thermal conductivity of FeSb_3 is only 1.14W/m K, which is 1/10 of that of CoSb_3. The filling of atoms does not result in a decrease in the lattice thermal conductivity of FeSb_3. This has changed the traditional understanding in the galactic cobalt system that filling atoms can significantly reduce the lattice thermal conductivity of galactic materials. The ultra-low lattice thermal conductivity in FeSb_3 is mainly due to the softening of the entire phonon spectrum. In particular, it is related to the softening of low frequency optical branch phonons associated with Sb-Sb covalent bonds in the structure. It is found that SnFe_4Sb_ (12) filled with high electronegativity element has ultralow intrinsic lattice thermal conductivity, which provides a new way to optimize the thermoelectric properties of galactic cobalt materials. We find that Sn atoms with high electronegativity have the central deviation phenomenon when filling the lattice vacancies into FeSb_3, which leads to the "Goldstone" phonon mode in FeSb_3. This is the main reason for the ultra-low intrinsic lattice thermal conductivity (0.69W/m K) of SnFe_4Sb_ (12) at room temperature. Furthermore, we design a double filling scheme, which applies the results obtained in SnFe_4Sb_ (12) research to the improvement of the thermoelectric properties of practical galactic cobalt ores, and provides a new way for the preparation of high performance galactic cobalt thermoelectric materials.
【学位授予单位】：吉林大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TB34

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