高压下典型双原子分子晶体的结构和性质的研究

发布时间：2018-05-01 10:08

本文选题：高压 + 固态氟　；参考：《吉林大学》2017年硕士论文

【摘要】：双原子分子晶体(H_2,N_2,O_2,F_2,Cl_2,Br_2,I_2)的高压性质一直是物理和化学界乃至地球和行星科学系长期关注的体系。其中,由于金属氢具有非常重要的意义,关于固态氢的高压理论和实验的研究就一直是高压科学研究的重点问题。然而因为氢分子内部的共价键和零点运动均较强,实验上至今未观测到固态氢的金属化现象。因此人们转向与固态氢类似的分子晶体体系,如卤族元素,以期获得研究金属氢的规律。本文以氢、氟、碘三种典型的双原子分子晶体为研究目标,用基于密度泛函理论的第一性原理计算方法对这三种体系的高压行为进行研究,希望对金属氢的研究有所启示。固态氢的高压理论和实验的研究已经相当多,但是也存在很多的争议,如氢的金属化现象。除此之外,目前高压理论研究提出很多固态氢的候选结构,但是只有氢的I相被X光直接证实,其他各相均没有直接的实验数据证实。基于此,我们考虑了不同赝势和有无范德瓦耳斯力的影响对已经报道的固态氢的多个结构采用CASTEP软件包进行几何优化,得到了固态氢分别在超软赝势考虑范德瓦耳斯力、超软赝势不考虑范德瓦耳斯力、模守恒赝势考虑范德瓦耳斯力、模守恒赝势不考虑范德瓦耳斯力这四种情况下的相图和相序。氟作为卤族元素的第一个元素和第一行可以形成双原子分子晶体的最后一个元素,与固态氢有很多相似之处。我们采用ELoc R软件对固态氟的晶体结构在0-100 GPa进行搜索。确定了低温常压下固态氟比较好的候选结构C2/c,提出一个高压新相Cmca。对C2/c和Cmca结构进行的电子性质计算表明这两个结构均为绝缘相。最后计算了C2/c和Cmca结构分别在1 GPa和20 GPa下的红外和拉曼,希望能为以后固态氟的实验提供理论依据。当前不论是理论还是实验对固态碘的研究已经相当充分,在理论和实验方面都发现了固态碘的金属化和压致解离等现象。根据理论和实验的报道,我们总结了固态碘的相序,并且优化了已经报道的Cmca和C2/m结构。15 GPa时,这两个结构均满足力学和动力学稳定性,且C2/m结构是金属相。其中C2/m结构有两种不同的分子内共价键长,且随压力的增加有分解的趋势。我们认为这种具有不同分子内键长的结构能够促进金属化和分子解离的发生。
[Abstract]:The high-pressure properties of the diatomic molecular crystal H2S / N2S / O / T / 2F / T / C / C / C / C / C / C / C / C / C / C / C / C / C / C / C / C / C / C / C / C / C / C / C / C / C / C / C / C / C / C / C / C Since metal hydrogen is of great significance, the research on high pressure theory and experiment of solid hydrogen has always been a key issue in high pressure science. However, the metallization of solid hydrogen has not been observed so far because of the strong covalent bond and zero point motion in the hydrogen molecule. Therefore, people turn to molecular crystal systems similar to solid hydrogen, such as halogen elements, in order to obtain the law of studying metal hydrogen. In this paper, three typical diatomic molecular crystals, hydrogen, fluorine and iodine, are used to study the high-pressure behavior of these three systems by using the first-principle calculation method based on the density functional theory. The high pressure theory and experiment of solid hydrogen have been studied much, but there are still many controversies, such as the metallization of hydrogen. In addition, many candidate structures of solid hydrogen have been proposed by high pressure theory, but only phase I of hydrogen has been confirmed directly by X-ray, and no direct experimental data have been obtained for the other phases. Based on this, we consider the effects of different pseudopotentials and Van der Waals forces on the geometry optimization of reported structures of solid hydrogen using CASTEP software package, and obtain that the van der Waals force of solid hydrogen is considered in super-soft pseudopotential respectively. The super soft pseudopotential does not consider van der Waals force, and the modulus conservation pseudopotential does not consider the phase diagram and phase order of the four cases. As the first element and the first line of halogen elements, fluorine can form the last element of diatomic molecular crystal, which has many similarities with solid hydrogen. We used ELoc R software to search for the crystal structure of solid fluorine at 0-100 GPa. The candidate structure C _ 2 / c for solid fluorine was determined at low temperature and atmospheric pressure, and a new high-pressure phase C _ (mcaa) was proposed. The calculation of the electronic properties of C _ 2 / c and Cmca structures shows that the two structures are insulating phase. Finally, the infrared and Raman spectra of C _ 2 / c and Cmca structures at 1 GPa and 20 GPa were calculated, respectively, in the hope of providing a theoretical basis for the experiment of solid fluorine in the future. At present, both theoretical and experimental studies on solid iodine have been fully studied. Metallization and pressure-induced dissociation of solid iodine have been found in both theory and experiment. According to the theoretical and experimental reports, we summarize the phase order of solid iodine, and optimize the reported structure of Cmca and C _ 2 / m. 15 GPa, the two structures satisfy the mechanical and kinetic stability, and C _ 2 / m structure is a metallic phase. The C _ 2 / m structure has two different intramolecular covalent bond lengths and has a tendency to decompose with the increase of pressure. We believe that this structure with different intramolecular bond lengths can facilitate metallization and molecular dissociation.
【学位授予单位】：吉林大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：O521.2

【相似文献】