發(fā)布時(shí)間：2018-03-26 06:38

  本文選題：高壓　切入點(diǎn)：第一性原理　出處：《吉林大學(xué)》2016年博士論文

【摘要】：壓力(強(qiáng))是與溫度和組分并列的控制凝聚態(tài)物質(zhì)結(jié)構(gòu)與性質(zhì)變化的熱力學(xué)參量。壓力作用下,物質(zhì)中原子間距會(huì)縮短,電子軌道重疊加劇,從而導(dǎo)致結(jié)構(gòu)相變;壓力也可以降低物質(zhì)的化學(xué)勢(shì)壘,促進(jìn)反應(yīng)的生成,進(jìn)而得到常壓下難以合成的新材料;壓力既可以使非超導(dǎo)體轉(zhuǎn)變?yōu)槌瑢?dǎo)體,也可以提高超導(dǎo)體的超導(dǎo)轉(zhuǎn)變溫度。目前,高壓已經(jīng)成為尋找新型功能材料的一種有效手段。1935年,Wigner和Huntington預(yù)言氫在壓力高于25 GPa時(shí),能夠從常溫常壓下的分子絕緣態(tài)轉(zhuǎn)變到類似堿金屬單原子金屬態(tài),即金屬氫。這主要?dú)w因于壓力作用下,物質(zhì)的帶隙會(huì)變窄,從而使得物質(zhì)從非金屬態(tài)變?yōu)榻饘賾B(tài)。之后在1968年,Ascroft指出因?yàn)闅漭^小的原子質(zhì)量及較高的電子態(tài)密度,極有可能是室溫超導(dǎo)體。然而到目前為止,盡管實(shí)驗(yàn)上已達(dá)到388萬大氣壓,但仍未獲得金屬氫。同時(shí),由于實(shí)驗(yàn)上的局限性,更高壓力的實(shí)現(xiàn)需要技術(shù)上的突破。2004年,Ascroft提出,富氫材料中的氫由于受到其它元素的“化學(xué)預(yù)壓縮作用”,有可能在目前實(shí)驗(yàn)所達(dá)到的壓力范圍內(nèi)表現(xiàn)出金屬化特性,從而可能成為潛在的高溫超導(dǎo)體候選材料。同時(shí)因?yàn)檫@些富氫化合物中本身含有大量的氫,所以它們可以成為潛在的儲(chǔ)氫材料。在本工作中,我們主要采用基于密度泛函理論(DFT)的第一性原理計(jì)算方法,結(jié)合晶體結(jié)構(gòu)搜尋技術(shù),對(duì)幾種新型氫化物在高壓下的晶體結(jié)構(gòu)、氫的存在形式、金屬化、超導(dǎo)電性及超導(dǎo)轉(zhuǎn)變機(jī)制等性質(zhì)進(jìn)行了系統(tǒng)地研究,獲得如下創(chuàng)新性研究成果:1)最近發(fā)現(xiàn)了新型硫氫化合物H3S在高壓下的超導(dǎo)轉(zhuǎn)變溫度達(dá)到203 K(-70oC),遠(yuǎn)遠(yuǎn)高于之前所報(bào)道的銅基超導(dǎo)體的超導(dǎo)轉(zhuǎn)變溫度(164 K)。因此,我們對(duì)同主族的釙氫化物在高壓下的結(jié)構(gòu)及超導(dǎo)電性進(jìn)行了詳細(xì)地研究,發(fā)現(xiàn)在高壓下出現(xiàn)四種穩(wěn)定配比的氫化物Po H、Po H2、Po H4和Po H6。除了Po H,氫是以氫原子形式存在外,其它配比Po H2、Po H4和Po H6中均出現(xiàn)了H2單元;電子行為的研究發(fā)現(xiàn),結(jié)構(gòu)Po H-P63/mmc、Po H2-Pnma、Po H4-C2/c和Po H6-C2/m均表現(xiàn)出金屬化特性;最后,計(jì)算了不同壓力下Po H-P63/mmc、Po H4-C2/c和Po H6-C2/m的超導(dǎo)轉(zhuǎn)變溫度,它們的Tc分別為0.14-0.65 K、41.1-47.2K和2.25-4.68K。2)常壓下銦氫化物無論是氣相態(tài)還是固相態(tài)均不存在,那么高壓下是否存在這種氫化物。通過第一性原理計(jì)算方法及結(jié)構(gòu)搜索軟件,研究了In-H體系在高壓下的穩(wěn)定配比、晶體結(jié)構(gòu)、電子性質(zhì)及超導(dǎo)電性。首次提出在高壓條件下,會(huì)出現(xiàn)兩個(gè)穩(wěn)定的配比In H3和In H5;In H3和In H5的穩(wěn)定晶體結(jié)構(gòu)中氫均是以H2或H3單元出現(xiàn);化學(xué)鍵的分析表明,H2或H3單元中H-H間是以共價(jià)鍵形式存在,而銦氫之間形成的是離子鍵,且電荷是從銦原子轉(zhuǎn)移至氫原子;能帶及態(tài)密度的計(jì)算表明結(jié)構(gòu)In H3-R-3和In H5-P21/m都是金屬,它們?cè)?00,150 GPa時(shí)的Tc分別為34.1-40.5 K和22.4-27.1 K。3)高壓實(shí)驗(yàn)上通過混合CH4和H2得到新型氫化物CH_4H_2,并且該化合物能夠穩(wěn)定存在到30 GPa,但是該化合物中氫原子的位置、電子性質(zhì)以及存在的相互作用還不很清楚。因此我們通過第一性原理方法對(duì)高壓下CH_4H_2的結(jié)構(gòu)及性質(zhì)等進(jìn)行了系統(tǒng)研究,給出了CH_4H_2的高壓相變序列:P-1→P212121→P21/C,相變壓力點(diǎn)分別為15.6 GPa和98.2 GPa,在這些結(jié)構(gòu)中,CH4和H2仍保持原來的分子狀態(tài)。另外通過模擬的拉曼光譜,驗(yàn)證了CH_4H_2中H2-鍵的硬化現(xiàn)象,同時(shí)也發(fā)現(xiàn)H2的取向無序現(xiàn)象。最后通過電子行為的研究發(fā)現(xiàn),在我們研究的壓力范圍內(nèi),CH_4H_2是絕緣體。4)鉑族金屬包括鉑(Pt)、鈀(Pd)、鋨(Os)、銥(Ir)、釕(Ru)、銠(Rh),其中鉑、鈀、銥和銠的氫化物在高壓下能夠穩(wěn)定存在。那么鋨氫化物和釕氫化物在高壓力下是否存在,如果存在,其高壓結(jié)構(gòu)、氫的存在形式、是否金屬化、超導(dǎo)電性等是怎樣的,這些問題都是值得研究的。因此我們對(duì)高壓下Os-H和Ru-H體系的結(jié)構(gòu)與性質(zhì)做了廣泛地研究。對(duì)于XHn(X=Os,Ru;n=1-8),我們首次預(yù)測(cè)出三種穩(wěn)定配比(XH、XH3和XH6);電子行為研究發(fā)現(xiàn),配比XH和XH3表現(xiàn)出金屬性,而XH6卻是半導(dǎo)體;有趣的是,我們?cè)赬H6中發(fā)現(xiàn)了H2單元,且H2單元中H-H是以很強(qiáng)的共價(jià)鍵形式存在;最后我們對(duì)金屬化的XH和XH3配比進(jìn)行了超導(dǎo)電性的研究,發(fā)現(xiàn)不同壓力點(diǎn)下結(jié)構(gòu)Fm-3m-Os H、Fm-3m-Ru H、Pm-3m-Ru H3和Pm-3n-Ru H3的超導(dǎo)轉(zhuǎn)變溫度分別為2.1 K、0.41 K、3.57 K和1.25K。5)我們對(duì)高壓下Hf H2體系的結(jié)構(gòu)與超導(dǎo)特性進(jìn)行了探索,發(fā)現(xiàn)低壓下最穩(wěn)定的相是I4/mmm,加壓至180 GPa時(shí)由I4/mmm結(jié)構(gòu)轉(zhuǎn)變?yōu)镃mma結(jié)構(gòu),250 GPa時(shí)又轉(zhuǎn)變?yōu)镻21/m結(jié)構(gòu),且兩次相變均屬于一級(jí)相變;能帶及態(tài)密度的計(jì)算發(fā)現(xiàn)這三個(gè)結(jié)構(gòu)均表現(xiàn)出金屬特性�；瘜W(xué)鍵的分析發(fā)現(xiàn),Hf H2是離子型晶體,電荷從鉿原子轉(zhuǎn)移至氫原子。最后我們?cè)诓煌瑝毫ο聦?duì)結(jié)構(gòu)I4/mmm、Cmma和P21/m進(jìn)行了超導(dǎo)電性的計(jì)算,得到它們的超導(dǎo)轉(zhuǎn)變溫度分別為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，

本文編號(hào)：1666772