【作者】 吴刚；

【导师】 崔田；

【作者基本信息】 吉林大学 ， 凝聚态物理， 2018， 博士

【摘要】 高压是一种极端条件,它能够非常有效地缩短原子间距离、增加相邻电子轨道重叠,进而改变原子(分子)间的相互作用和电子结构,形成常规条件下难以形成的具有新结构与新性质的高压新相。对电子结构最简单的氢元素而言,理论预测其在极高压力下会转变为金属相,更重要的是金属氢被认为极有可能具有接近室温的超导转变温度。因此金属氢是高压科学中受到最广泛关注与研究的课题之一,被喻为高压物理学界的圣杯。对金属氢的探索至今已有八十余年,直到最近哈佛大学研究人员在Science上报道,在495万大气压(GPa)条件下观察到了金属氢。但是这一实验结果存在较大争议,还需要进一步验证。因此人们从其他途径开展研究以期在相对较低的压力条件下获得一些对于金属氢的规律性认识,这种研究主要包括两方面:一方面研究是基于2004年Ashcroft提出的化学预压效应,即在氢的晶格中引入其他较大的原子,引入的原子会对原有的氢原子产生挤压效应,这种挤压效应会显著降低氢金属化的压力。另一方面是从理化性质与氢元素非常相似的卤族元素入手,研究卤族元素及其化合物的金属化现象及高压相变机制,对理解高压下氢分子的解离与金属化有重要帮助。本文的选题基于上述提到的两方面研究。对碱土金属Mg-H与Ca-H两体系进行了高压研究,研究包括单质钙、镁与氢气在高温高压下形成的非常规比例氢化物,以及Mg-H体系唯一的常规配比化合物Mg H2高压结构相变,这一系列研究有助于加深对化学预压效应的理解,为金属氢的探索提供指引。另外选取典型的稀有气体卤化物二氟化氙,我们结合拉曼散射光谱,吸收光谱,同步辐射等高压实验技术,结合第一性原理计算模拟,对其高压下的结构相变及金属化现象进行了系统的研究,以期获得对有助于金属氢研究的物理图像。具体我们获得了如下创新性结果:1.单质钙与氢在48.5GPa,1900K的条件下形成了一种新型氢化物。XRD与拉曼结果表明这一氢化物是前人理论预测的Ca H4(空间群I4/mmm)。这是首次在实验上合成出钙的非常规配比氢化物,并且在目前所有实验上合成出的非常规配比金属氢化物中,Ca H4是唯一一个金属:氢比例为1:4的的氢化物。利用三阶Birch-Murnaghan物态方程拟合得到了Ca H4的零压体弹模量为25.7(4)GPa。卸压过程的拉曼测试表明Ca H4会在28.7GPa发生分解。这一Ca H4的每个原胞中含有两个H-离子以及一个H2单元,该H2单元的长度略长于纯氢中的H2分子的长度。在50GPa时Ca H4中H2单元的长度近似于纯氢分子在220GPa时的长度,即Ca H4中的H2单元类似于更高压力条件下的氢分子,并且该H2单元的长度随压力增加而增大,体现出解离的趋势。因此H2单元可能在较低的压力就发生解离,Ca H4可能成为一种的金属氢的载体材料。这对进一步探索化学预压效应下氢原子的金属化有重要意义。2.对Mg-H体系的研究分两方面,分别对常规配比Mg H2的结构研究和非传统比例镁氢化合物的合成研究。通过高压同步辐射XRD研究确定了Mg H2的高压相序。二氢化镁在11.2GPa由α相与γ相共存转变为Pbc21型结构,进而在17GPa变为Pnma结构。解决了实验上对于Mg H2高压结构相变所存在的分歧。两次相变分别对应13%与10%的体积塌缩。为探索单质镁与氢在高温高压下形成反常配比氢化物提供了对比数据。单质镁与氢的高温高压研究中,分别在42GPa与55GPa时对样品进行激光加热至约2000K。镁在加热后失去原有的金属光泽并发生延展,但XRD与拉曼的结果均表明镁与氢在两次加热后形成的都是常规配比的二氢化镁(空间群:Pnma)。形成更高氢含量的非常规氢化物还需要更高的压力与温度条件。3.前人报导二氟化氙在高压下会发生金属化现象,同时伴随较丰富的结构相变,并且目前的研究关于其高压相变情况始终没有一致的结论。我们利用Raman光谱,吸收光谱,XRD等高压技术结合DFT计算模拟对Xe F2高压下的结构相变与带隙变化进行了全面的研究。发现Xe F2的I4/mmm对称性并没有如理论所预测的保持到82GPa,而是在28GPa其对称性就降低为Immm型,相应的在拉曼光谱中观测到原本I4/mmm结构对应的二重简并峰发生了退简并现象,以及吸收光谱中探测到I4/mmm结构由于较高的结构对称性而被限制的电子跃迁。进而在59 GPa,由于Xe F2中F-Xe-F的几何构型发生了弯曲,二氟化氙转变为理论预测中100 GPa之后才会出现的Pnma型结构,由于压腔内的非静水压条件导致了这一结构提前出现。基于Immm结构与Pnma结构的拉曼模拟与XRD模拟结果都与我们的实验结果符合良好。我们认为压腔内的非静水压条件是造成目前对于Xe F2的实验研究与理论计算之间存在分歧的主要原因。由吸收光谱结果得到了二氟化氙的带隙在压力下的变化,发现在82 GPa时,二氟化氙仍具有约1.83 e V的带隙,我们推测带隙闭合的压力约为152 GPa。4.在试验方法上,自主搭建了一套氢气封装系统;并对激光加热样品装填方法进行优化,利用优化后的方法合成了ReN2。更多还原

【Abstract】 The extreme conditions of high pressure can decrease the distance between atoms signally,leading to stronger interactions among atoms.Pressure would have a profound effect on the nature of the material,accompanying with formation of high pressure phase with new structure and new property,that is absent in the normal conditions.Hydrogen,as the simplest element,is predicted to become metallic phase under ultrahigh pressure.According to the prediction,this metallic phase is also one kind of room temperature superconductor.So there have been wide investigations on metallic hydrogen,which is discribed as “the holy grail” of high pressure physics.The exploration of metal hydrogen has continued for more than eighty years so far.Recently,one research group of Harvard university published their achievement on Science,declaring that they have detected the sigh of metallic hydrogen under the pressure of 495 GPa.However,this controversial result still needs further verification,and this high metallic pressure has increased the difficulty of high-pressure further exploration seriously.So people carry out some researches in another way in order to obtain some regular understanding of metallic hydrogen at lower pressure condition.These researches mainly include two aspects.The first approach is based on the chemical pre-compression effect proposed by Ashcroft at 2004.The heavier atom introduced in the lattice of hydrogen will produce compression effect on the hydrogen atoms,which is predicted to decrease the metallization pressure of hydrogen.The second is researches on halogen elements whose physical and chemical properties are similar to hydrogen elements.Studies on the metallization of halogen elements and their compounds and the mechanism of high pressure transition will be helpful to understand the dissociation and metallization of hydrogen under high pressure.The topic of this paper is based on the two aspects of researches mentioned above.We have carried out research on the system of calcium-hydrogen and magnesium-hydrogen,which including behaviors of Mg+H2 and Ca+H2 under high pressure high temperature conditions and high pressure structural transition of the ordinary stoichiometric Mg H2.This systematic research will be helpful to understand the chemical pre-compression effect,and guide the exploration of metallic hydrogen.In addition,xenon difluoride is a typical noble gas fluoride.In order to obtain the physical image helpful to study on metallic hydrogen,we have adopted high pressure experiment technique and first principle calculations to investigate the high pressure structure transition and metallic phenomenon of Xe F2.In summary,we have get the following innovative results.1.Calcium polyhydride is synthesized from elements at 50 GPa and temperature above 1900 K experimentally for the first time.By analyzing results of Raman and XRD measurement up to 70 GPa,This polyhydride is inferred to be the untraditional stoichiometry Ca H4 which is predicted to be the ground state of Ca-H system at 50 GPa in former theoretical study.Among all of the metal hydrides with unconventional stoichiometry observed under high pressure,Ca H4 is the only one with the metal: hydrogen ratio of 1:4.The zero-pressure bulk modulus B0 of Ca H4,obtained from fitting to a Birch-Murnaghan equation of state is 25.7(4)GPa.The decomposition of Ca H4 is observed at 28 GPa in Raman spectra.The crystal structure of Ca H4 with I4/mmm symmetry includes two ionic hydrogen and one elongate H2 unit,which has the similar length with hydrogen molecular at higher pressure.According to the variation trend of H2 unit in Ca H4,this unit is suggested to dissociate into metallic phase at more moderate pressure than pure hydrogen.In this respect,Ca H4 could be served as the chemical pre-compression material,which is regarded as the carrier of metallic hydrogen.This study will attribute to realizing chemical pre-compression effect and exploring metallic hydrogen.2.Researches on Mg-H system are in two two aspect,including the study on high pressure structure phase transition of Mg H2 and the synthesis of magnesium hydride with unconventional stoichiometry.We have confirmed the phase sequence of magnesium hydride under pressure.Mg H2 is stable as the coexist of α phase and γ phase before 11.2 GPa,after which it transforms into a Pbc21 phase.The Pnma phase takes the place of Pbc21 phase at 17 GPa.These two phase transitions are corresponding with 13% and 10% volume collapse respectively.Our results solve the controversy on high pressure structures of Mg H2,and provide comparative data for the following exploration on magnesium and hydrogen.In the study on magnesium and hydrogen under high pressure and high temperature,the sample is heated to 2000 K by laser at 42 GPa and 55 GPa.There is change on the morphology of magnesium after laser heating.Results of Raman and XRD both indicate the formation of magnesium hydride(Mg H2).The synthesis of magnesium hydride with unconventional stoichiometry still need higher pressure and temperature.3.It has been reported that xenon difluoride will undergo metallization transition under high pressure,accompanying with abundant structural phase transition.But both crystal structure and phase transition sequence of Xe F2 remain controversial under high pressure.We have investigated the structural phase transition and band gap change of Xe F2 by means of Raman spectra,absorption spectra and synchrotron XRD,combined with DFT calculations.The I4/mmm structure which is predicted to be stable before 82 GPa,collapse into Immm at 28 GPa.Simultaneously,the double degenerate Raman peak slip into two individual peaks,and a symmetry forbidden electron transition is observed in absorption spectra.At 59 GPa the predicted high pressure phase(Pnma structure)is definitely confirmed in experiment for the first time.In our study,non-hydrostatic conditions significantly reduce the phase transition pressure and the lattice symmetry.We propose that non hydrostatic effect in sample chamber is response for the discrepancy between the experimental research and theoretical calculations on Xe F2.A reduction of the band gap is indicated by the optical absorption experiment,accompanying with the sample color getting dark as pressure increase.The metallization pressure is estimated to be 152 GPa.4.In the experimental method,we have built a hydrogen gas loading system independently.The method of sample loading in laser heating experiment has been optimized,which has been used to synthesis Re N2.更多还原