碱金属过氧化物和超氧化钠的高压相变和电子性质研究

发布时间：2018-05-10 14:27

本文选题：碱金属过氧化物 + 超氧化钠　；参考：《东北师范大学》2017年硕士论文

【摘要】：同一种物质有着不同的晶体结构,由于结构决定性质,所以不同的结构对其材料的性能有较大的影响。在此基础上,对结构相变及其相变机理的研究在地球科学、材料科学和化学物理等许多领域都具有重要意义。压力作为除了温度外的另一一维条件,对于物质的各种物理和化学性质都有着不用程度的影响。压力作用于物质上,可以有效减小物质中原子或分子间的距离,对物质晶体结构、能带结构及其电子轨道结构进行调制。高压相变是物质在高压环境下产生的相变。由于在高压作用下,原子间距缩短,由电子和点阵的相互作用引起的系统不稳定性,进而导致物质内部能量状态发生变化。在高压或高温高压条件下,晶体结构、物质中原子排列及电子结构会发生变化,并伴随着物理性质的变化,例如从绝缘体变为金属、体积和电阻发生很大程度改变等,这种现象称为高压相变。本论文采用第一性原理计算了高压下碱金属(Li,Na,K和Rb)过氧化物及超氧化钠的结构和电子性质,主要工作有:(1)采用第一性原理在0~100 GPa高压下对碱金属过氧化物的结构相变、晶格动力学和电子性质进行了计算及讨论。该工作预测了Li2O2、Na2O2、K2O2和Rb2O2的一级相变压力点分别为84、28、7和6 GPa,说明一级相变压力随着碱金属电负性的减小而减小。这些预测的高压相均具有热力学稳定性,不会分解为碱金属氧化物和O2或者碱金属和O2。同时,在所研究的压力范围内过氧根(O22-)始终保持不变。声子计算也证实了这些结构具有动力学稳定性,碱金属过氧化物的带隙随着压力的增大而增大。(2)采用第一性原理在0~20 GPa高压下对超氧化钠的结构和电子性质进行了计算和讨论。研究发现正交晶系空间群为Pnnm的常压结构在4.6 GPa压力下相变为另一个正交晶系的Immm结构,并且随后在6.7 GPa相变为四方晶系的P4/mbm结构。高压引起的相变主要由多面体的密堆积和更高的配位数具有更稳定的结构所导致。超氧根(O2-)在整个研究的压力范围内保持了不变。电子能带和态密度展示了这些结构都具有半金属磁性。
[Abstract]:The same material has different crystal structure, because the structure determines the property, so the different structure has great influence on the properties of the material. On this basis, the study of structural phase transition and its mechanism is of great significance in many fields such as earth science, material science and chemical physics. As a one-dimensional condition besides temperature, pressure has no effect on the physical and chemical properties of matter. The pressure acting on the matter can effectively reduce the distance between atoms and molecules, and modulate the crystal structure, energy band structure and electron orbital structure of the matter. High pressure phase transition is the phase transition of matter under high pressure environment. Due to the reduction of the atomic distance under high pressure, the instability of the system caused by the interaction between electrons and lattice leads to the change of the internal energy state of the matter. Under high pressure or high temperature and high pressure, the crystal structure, atomic arrangement and electronic structure in matter will change, accompanied by changes in physical properties, such as changing from insulator to metal, volume and resistance, etc. This phenomenon is called high pressure phase transition. In this paper, the structure and electronic properties of the alkali metal Li-nai K and RB) peroxides and the superoxide sodium under high pressure are calculated by first principle. The main work includes the structure phase transition of the alkali metal peroxides at 0 ~ 100 GPa high pressure by using the first principle. Lattice dynamics and electronic properties are calculated and discussed. The first order phase transition pressure of Li _ 2O _ 2, Na _ 2O _ 2, K _ 2O _ 2 and Rb2O2 is 844 ~ 28 ~ 7 and 6 GParespectively, which indicates that the first-order transition pressure decreases with the decrease of electronegativity of alkali metals. These predicted high-pressure phases are thermodynamically stable and will not be decomposed into alkali metal oxides and O2 or alkali metals and O _ 2. At the same time, the peroxy radical O 22) remains unchanged in the studied pressure range. The phonon calculations also show that these structures have dynamic stability. The band gap of alkali metal peroxides increases with the increase of pressure. The structure and electronic properties of sodium superoxide are calculated and discussed by first principle at 0 ~ 20 GPa high pressure. It is found that the normal pressure structure of the space group of the orthorhombic system is Pnnm at 4.6 GPa and the Immm structure of another orthorhombic system is transformed into the P4/mbm structure of the tetragonal system at 6.7 GPa. The phase transition induced by high pressure is mainly caused by the dense stacking of polyhedron and the more stable structure of the coordination number. Superoxide O _ 2) remained constant throughout the pressure range of the study. The electron band and density of states show that these structures are semimetallic magnetic.
【学位授予单位】：东北师范大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：O611.62

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