高压下几种新型氢化物结构与性质的理论研究

发布时间：2018-03-26 06:38

  本文选题：高压　切入点：第一性原理　出处：《吉林大学》2016年博士论文

【摘要】：压力(强)是与温度和组分并列的控制凝聚态物质结构与性质变化的热力学参量。压力作用下,物质中原子间距会缩短,电子轨道重叠加剧,从而导致结构相变;压力也可以降低物质的化学势垒,促进反应的生成,进而得到常压下难以合成的新材料;压力既可以使非超导体转变为超导体,也可以提高超导体的超导转变温度。目前,高压已经成为寻找新型功能材料的一种有效手段。1935年,Wigner和Huntington预言氢在压力高于25 GPa时,能够从常温常压下的分子绝缘态转变到类似碱金属单原子金属态,即金属氢。这主要归因于压力作用下,物质的带隙会变窄,从而使得物质从非金属态变为金属态。之后在1968年,Ascroft指出因为氢较小的原子质量及较高的电子态密度,极有可能是室温超导体。然而到目前为止,尽管实验上已达到388万大气压,但仍未获得金属氢。同时,由于实验上的局限性,更高压力的实现需要技术上的突破。2004年,Ascroft提出,富氢材料中的氢由于受到其它元素的“化学预压缩作用”,有可能在目前实验所达到的压力范围内表现出金属化特性,从而可能成为潜在的高温超导体候选材料。同时因为这些富氢化合物中本身含有大量的氢,所以它们可以成为潜在的储氢材料。在本工作中,我们主要采用基于密度泛函理论(DFT)的第一性原理计算方法,结合晶体结构搜寻技术,对几种新型氢化物在高压下的晶体结构、氢的存在形式、金属化、超导电性及超导转变机制等性质进行了系统地研究,获得如下创新性研究成果:1)最近发现了新型硫氢化合物H3S在高压下的超导转变温度达到203 K(-70oC),远远高于之前所报道的铜基超导体的超导转变温度(164 K)。因此,我们对同主族的钋氢化物在高压下的结构及超导电性进行了详细地研究,发现在高压下出现四种稳定配比的氢化物Po H、Po H2、Po H4和Po H6。除了Po H,氢是以氢原子形式存在外,其它配比Po H2、Po H4和Po H6中均出现了H2单元;电子行为的研究发现,结构Po H-P63/mmc、Po H2-Pnma、Po H4-C2/c和Po H6-C2/m均表现出金属化特性;最后,计算了不同压力下Po H-P63/mmc、Po H4-C2/c和Po H6-C2/m的超导转变温度,它们的Tc分别为0.14-0.65 K、41.1-47.2K和2.25-4.68K。2)常压下铟氢化物无论是气相态还是固相态均不存在,那么高压下是否存在这种氢化物。通过第一性原理计算方法及结构搜索软件,研究了In-H体系在高压下的稳定配比、晶体结构、电子性质及超导电性。首次提出在高压条件下,会出现两个稳定的配比In H3和In H5;In H3和In H5的稳定晶体结构中氢均是以H2或H3单元出现;化学键的分析表明,H2或H3单元中H-H间是以共价键形式存在,而铟氢之间形成的是离子键,且电荷是从铟原子转移至氢原子;能带及态密度的计算表明结构In H3-R-3和In H5-P21/m都是金属,它们在200,150 GPa时的Tc分别为34.1-40.5 K和22.4-27.1 K。3)高压实验上通过混合CH4和H2得到新型氢化物CH_4H_2,并且该化合物能够稳定存在到30 GPa,但是该化合物中氢原子的位置、电子性质以及存在的相互作用还不很清楚。因此我们通过第一性原理方法对高压下CH_4H_2的结构及性质等进行了系统研究,给出了CH_4H_2的高压相变序列:P-1→P212121→P21/C,相变压力点分别为15.6 GPa和98.2 GPa,在这些结构中,CH4和H2仍保持原来的分子状态。另外通过模拟的拉曼光谱,验证了CH_4H_2中H2-键的硬化现象,同时也发现H2的取向无序现象。最后通过电子行为的研究发现,在我们研究的压力范围内,CH_4H_2是绝缘体。4)铂族金属包括铂(Pt)、钯(Pd)、锇(Os)、铱(Ir)、钌(Ru)、铑(Rh),其中铂、钯、铱和铑的氢化物在高压下能够稳定存在。那么锇氢化物和钌氢化物在高压力下是否存在,如果存在,其高压结构、氢的存在形式、是否金属化、超导电性等是怎样的,这些问题都是值得研究的。因此我们对高压下Os-H和Ru-H体系的结构与性质做了广泛地研究。对于XHn(X=Os,Ru;n=1-8),我们首次预测出三种稳定配比(XH、XH3和XH6);电子行为研究发现,配比XH和XH3表现出金属性,而XH6却是半导体;有趣的是,我们在XH6中发现了H2单元,且H2单元中H-H是以很强的共价键形式存在;最后我们对金属化的XH和XH3配比进行了超导电性的研究,发现不同压力点下结构Fm-3m-Os H、Fm-3m-Ru H、Pm-3m-Ru H3和Pm-3n-Ru H3的超导转变温度分别为2.1 K、0.41 K、3.57 K和1.25K。5)我们对高压下Hf H2体系的结构与超导特性进行了探索,发现低压下最稳定的相是I4/mmm,加压至180 GPa时由I4/mmm结构转变为Cmma结构,250 GPa时又转变为P21/m结构,且两次相变均属于一级相变;能带及态密度的计算发现这三个结构均表现出金属特性。化学键的分析发现,Hf H2是离子型晶体,电荷从铪原子转移至氢原子。最后我们在不同压力下对结构I4/mmm、Cmma和P21/m进行了超导电性的计算,得到它们的超导转变温度分别为47-193 m K、5.99-8.16K和10.62-12.8 K。
[Abstract]:Pressure (strong) and control of temperature and component condensation of thermodynamic parameter changes of structure and properties of matter. The pressure in the material will shorten the distance between atoms, electron orbital overlap intensified, leading to the structure transformation; pressure can also reduce the chemical barrier material, promote the formation of reaction, and obtain new materials under atmospheric pressure it is difficult to synthesis; pressure can make non superconductor for superconductors, but also can improve the temperature superconductors. At present, high pressure to find new functional materials has become an effective means of.1935, Wigner and Huntington in the predicted hydrogen pressure is higher than 25 GPa, from molecular at ambient temperature and insulation transition is similar to that of alkali metal atom metal state and metal hydrogen. This is mainly attributed to the pressure of the narrow band gap material, so that the materials from the non metallic state into metal state. In 1968, Ascroft pointed out that because the electron density of atomic mass and higher hydrogen less, is likely to be room temperature superconductors. However, so far, although the 3 million 880 thousand atmospheric pressure has reached the experiment, but has yet to get metal hydrogen. At the same time, due to the limitation of the experiment, the higher pressure to realize the technology the breakthrough of.2004 years, Ascroft proposed that the hydrogen in the hydrogen rich material due to other elements of the "chemical pre compression, pressure range may have reached in the present experimental exhibits properties of metallization, which may become a potential candidate materials for high temperature superconductors. At the same time because of the hydrogen rich compounds in itself contains a lot of the hydrogen, so they can become a potential hydrogen storage materials. In this work, we mainly based on the density functional theory (DFT) calculation method of the first principle, combined with the crystal structure of several search technology The crystal structure of a novel hydride under high pressure, exist in the form of hydrogen metallization, properties of superconductivity and superconducting transition mechanism has been studied, obtained achievements are summarized as follows: 1) recently discovered a new type of compound H3S under high pressure superconducting transition temperature reaches 203 K (-70oC), the superconducting transition temperature is much higher than previously reported copper based superconductors (164 K). Therefore, we are on the same main group of polonium hydride structure under high pressure and superconductivity were studied in detail, found that there are four stable ratio under high pressure Po H2, hydride Po H, Po H4 Po H6. and Po H in addition, hydrogen exists in hydrogen atoms form, other ratios of Po H2, Po H4 and Po were found in H6 H2 research unit; electronic behavior found that the structure of Po H-P63/mmc, Po H2-Pnma, Po H4-C2/c and Po H6-C2/m showed metal characteristics; finally, calculated Under different pressure of Po H-P63/mmc, Po H4-C2/c and Po superconducting transition temperature H6-C2/m, their Tc were 0.14-0.65 K, 41.1-47.2K and 2.25-4.68K.2) under normal pressure both indium hydride gas phase or solid state does not exist, then under high pressure and the existence of this hydride. The calculation method and the structure of the search software through the first principle. Stable ratio of In-H system were investigated under high pressure crystal structure, electronic properties and superconductivity. First proposed in under the condition of high pressure, there will be two stable ratio of In H3 and In H5; In H3 and In H5 stable crystal structure of hydrogen were in the H2 or H3 unit; analysis of chemical bond show that the H2 or H3 unit in H-H is existed in the form of covalent bond, and the formation of indium hydrogen between the ionic bond, and the charge is transferred to the indium atoms from the hydrogen atom; energy band calculation and density of states show that In H3-R-3 and In H5-P21/m structure They are metal, at 200150 GPa Tc 34.1-40.5 K and 22.4-27.1 K.3 respectively) on the high-pressure experiments by mixing CH4 and H2 new hydride CH_4H_2 and the compound is stable to 30 GPa, but the hydrogen atom in the compound position, electronic properties and the interaction is still not clear. Therefore we through first principle methods on structure and properties of CH_4H_2 under high pressure were studied. The high pressure phase transition sequence are given CH_4H_2: P-1 - P212121 - P21/C, the phase transition pressure point were 15.6 GPa and 98.2 GPa, in the structure, CH4 and H2 still maintain the original molecular state. In addition by Raman spectrum simulation, verified the hardening of the CH_4H_2 bond of H2-, also found that orientational disorder phenomenon of H2. Finally through the research on the electronic behavior found in the pressure range of our study, CH_4H_2 insulation 浣，

本文编号：1666772