【作者】 姜树清；

【导师】 崔田；

【作者基本信息】 吉林大学 ， 凝聚态物理， 2014， 博士

【摘要】 氢是元素周期表中的第一号元素，核外只含有一个电子,在所有元素中具有最简单的电子结构。由于其电子结构的独特性，氢元素既可以归为IA族碱金属元素，又被认为具有VIIA卤族元素类似的性质。对于氢的研究，不仅具有重要的学术意义，更具有深远的实际应用价值。一方面，金属氢是二十一世纪最为重要的十大物理问题之一，氢被认为在压力作用下会由绝缘体转变为金属，并且金属氢是一种潜在的室温超导体，在实际应用中有望使超导体这一应用广泛的功能材料摆脱低温的限制。然而，现有实验条件下尚未观测到金属氢的确切证据。鉴于直接对氢加压实现金属化所需的压力过高，理论研究结果认为含氢化合物中其他元素对氢原子会产生一种化学预压作用，这会大大降低氢金属化所需的压力。因此，富氢材料的高压研究为金属氢的研究提供了另一条途径。另一方面，氢通常的单质形态是氢气，是一种由双原子分子组成的具有极高燃烧热能的气体。氢能源因为其高能量密度、可循环利用且绿色无污染等良好的特性而被认为是一种新型能源材料，人们期望氢能源能够减少人们对于传统化石燃料的依赖，从而降低日益严重的CO2污染对环境造成的压力。除却传统以C和H元素为主的化石燃料外，N和H元素组成的化合物也因为其良好特性引起人们广泛兴趣。首先，这类材料完全燃烧产物为N2和H2O，不会对环境产生污染；其二，N-N键和H-H键的键能都非常高,分别基于N元素和H元素的聚氮和富氢材料都已经被广泛研究和应用，若两者结合，性能将更加优异。此外，氢键的存在对含氢材料高压下的结构和性质发挥着重要的作用，在与N原子电负性相近的O，F，Br和Cl等元素与H元素结合形成的化合物中，都在较低压力下发现了氢键对称化这一特殊现象。然而在以NH两种元素形成的化合物中却尚未观测到氢键对称化的确切证据。因此研究这一类型氢键的对称化现象对深入认识物质中原子之间的基本相互作用力具有非常意义。在本文中，我们选取典型含氢小分子联氨（N2H4）及联氨盐水合联氨（N2H4H2O），盐酸联氨（N2H4HCl）和溴酸联氨(N2H4HBr)作为研究体系。采用原位高压实验测量技术，辅助以空间群分析、第一性原理计算，首次对典型含氢小分子联氨及联氨盐高压下的结构、氢键以及材料稳定性进行了详细研究。研究结果使我们对这一类材料的高压行为有了深入了解，并在一定程度上揭示了振动模式之间复杂的耦合、共振作用和氢键对称化等特殊的高压现象，对我们实现氢的金属化以及富氢材料进一步加氢设计具有一定的借鉴作用。具体结果如下：（1）高压下N2H4的结构变化及氢键的研究联氨是一种由N元素和H元素形成的典型含氢小分子，其含氢量高达12.6wt%，常与液态氢混合作为一种混合燃料应用在航空航天领域。前人对固态联氨的研究多集中在低温条件下，高压下相变序列以及结构尚未确定。通过原位高压拉曼光谱测量和同步辐射XRD技术，对联氨进行高压实验研究分别至46.5和33.0GPa。实验结果显示液态联氨在1.2GPa发生固化，通过Raman峰指认，群论分析和XRD光谱精修，首次确定第一个固态相I的结构为P21。随着压力增加，相I在2.4GPa进一步转变为相II，光谱的变化显示这可能是由于联氨分子之间氢键的形成导致的。在18.4GPa，拉曼光谱中NH2基团的Deformational振动峰发生了从红移到蓝移的转变，标志着联氨发生等结构相变，结构转换为相III。通过对Deformational振动模式的构型及其周围成键环境的分析，首次对分叉型氢键在压力作用下的变化进行了分析，提出分叉型氢键中两个分枝键长和键角随压力的调整是等结构相变的相变机制。此外，在20.6GPa以上，NH伸缩振动振动峰随压力增加不断向低波数移动，同时峰强减弱，峰宽变宽，至约30GPa彻底消失。这种明显的模式软化行为正是由于氢键的增强引起的，是氢键对称化过程中的一个重要光谱特征。通过DMP理论对这一现象分析指出，联氨中N-H…N这种氢键模型的对称化可能发生在大约60GPa。（2）N2H4H2O的高压行为研究我们利用原位高压Raman光谱测量和同步辐射XRD实验技术对水合联氨（N2H4H2O）首次进行了高压下结构变化的实验测量。实验结果显示液态水合联氨在3.2GPa固化为相I。拉曼光谱特征显示在相I中H2O中的H原子被N2H4中的不饱和N原子吸引形成NH+3离子。样品在7.2GPa进一步发生变化，结构由相I变为相II，光谱分析认为这次相变是N2H4分子的扭转引起的。第一次Raman光谱测量至36.0GPa，结果显示大多数OH对称伸缩峰在20GPa以上红移消失不见，此压力点与H2O中对应模式的软化压力相近，表明O-H…N类型的氢键对称化也可能发生在较低压力范围内。在整个加压过程中，没有观测到杂质N2H4和H2O的拉曼信号，由此证明1：1组份的N2H4和H2O其高压结晶物为纯净的一水联氨。在卸压过程中，压力降至2.3GPa时光谱发生剧烈变化，通过对比和指认，发现此时样品为纯联氨的相I结构，一水联氨发生分解，水的信号消失。第二次Raman光谱测量至13.3GPa，结果显示在卸压至1.9GPa时样品发生分解，水的Raman信号消失，测得为纯联氨。同步辐射XRD光谱显示样品在40.4GPa左右发生了分解，卸至1.5GPa时光谱未再发生进一步变化。（3）高压下N2H4HCl的结构变化及氢键的研究我们利用原位高压拉曼光谱测量技术和同步辐射XRD衍射实验技术，首次对固体N2H4HCl进行高压实验研究分别至39.5和24.6GPa。通过对测得同步辐射XRD光谱进行全谱拟合，确定常温常压条件下固态N2H4HCl相I结构为C2/c。在相I中N2H+5离子交错排列，通过N-H…N模式的氢键连接成链。从相I到相II的结构转变发生在7.3GPa，在相II中Cl-离子开始参与形成氢键，导致NH伸缩振动模式发生明显的软化红移并与低频区振动模式发生费米共振。通过对主要晶格振动峰和光学振动峰随压力移动频率的计算和分析，发现NH伸缩振动峰表现出缓慢的峰位移动，通过分析我们认为主要是由氢键增强对压力效用的抵消造成的。压力高于19.8GPa时，相II转变为相III，此相中费米共振现象消失，共振过程完成，证明部分N-H…Cl模式的氢键可能发生了对称化。样品结构直到最高压力39.5GPa未再发生变化。在不同的结构中，Deformational模式振动峰表现出截然不同速率的反常红移，这种频移速率的变化也是相变的证据之一。（4）高压下N2H4HBr的结构和氢键对称化研究我们利用原位高压Raman光谱测量，红外光谱（IR）和同步辐射XRD实验技术对固态N2H4HBr进行高压研究，压力分别至65.4，23.9和27.0GPa。通过相I的Raman光谱特征峰指认和XRD衍射光谱的全谱拟合，确定常温常压条件下相I结构为C2/c。相I到相II的转变发生在6.6GPa。Raman光谱分析表明Br-离子参与形成氢键导致氢原子位置的移动是这次相变的根本原因。通过对相II中XRD测量光谱的指标化，确定相II结构为P1。在12.7GPa，样品结构转变为相III，在相III中，软化的NH伸缩振动模式与低频区振动模式发生了明显的费米共振，开始于12.7GPa，结束于24.5GPa。在34.5GPa，结构进一步相变为IV相，部分N-H…Br类型的氢键在此相中发生了氢键对称化。相IV稳定至最高压力65.4GPa未再变化，卸压后结构回到相I，证明相变是可逆相变。更多还原

【Abstract】 Hydrogen is the first element in the periodic table of the elements, which has thesimplest atomic structure in all elements with only one electron outside the nucleus.Owing to the special electronic structure, the hydrogen can be recognized as an alkalimetal element in IA family, and also one member of the halogen elements in VIIAfamily. The study of hydrogen not only has academic significance, but also is crucialin practical application. On one hand, Metallic hydrogen is one of ten major physicalproblems in21st century. It is predicted that the hydrogen can become an alkali metalunder extreme compression. Furthermore, the metal hydrogen was recognized as aroom temperature superconductor, which may get rid of the low-temperaturelimitations in practical application. However, the alkali metal has not achieved in theexperiment so far. Owing to the ultrahigh pressure in pressurizing pure hydrogendirectly, it is proposed that the role of chemical pre-compression in the hydrogen-richmaterials could lower the pressure of metallization. Therefore, researching thebehavior of hydrogen atom in the hydrogen-rich materials under high pressureprovides a shortcut in hydrogen metallization. On the other hand, the hydrogen gas isthe elemental form of hydrogen, which is composed of a diatomic molecule withextremely high energy. Hydrogen gas is viewed as a new energy source vital to thefuture economy because of its high energy density and pollution-free combustionproduct. It is expected to reduce the dependence of our economy on fossil fuels andalleviate the ever-worsening CO2pollution that threatens our environment. Apart fromthe traditional fossil fuel consisted of C and H elements, the compounds composed ofN and H elements have attracted extensive interest because of its good properties.Firstly, the complete combustion products of these materials are N2and H2O, which isno pollution to environment. Furthermore, both the NN and HH bonds energy isrelatively high, the polynitrogen and hydrogen-rich compounds have been researchedand applied extensively. The new materials based on the N and H atoms would have higher energy density. In addition, the hydrogen-bond is vital to the structure andproperty of hydrogen-bonding materials under pressure. The hydrogen-bondsymmetrization has been obsvred in the hydrides with O, F, Cl and Br. However, it isnot clear in the hydride with N atom, which has similar electronegativity with thementioned atoms above. The research on such hydrogen-bond would give a deeperunderstanding on the basic interaction between atoms.We have focused on the typical hydrogen-rich molecules hydrazine (N2H4) andhydrazine salts (N2H4H2O, N2H4HCl and N2H4HBr) as the main objects. Thehigh-pressure structure, hydrogen-bond and stability of hydrazine and hydrazine saltshave been firstly researched by the in situ high pressure experiment, space groupanalysis and first principles calculation. The results give a deep understanding of thehigh-pressure behavior in such materials, and show a series of phenomenons ofcoupling between vibrations, hydrogen-bond symmetrization and so on, which have acertain reference on hydrogen metallization and hydrogenation of hydrogen-richmolecules. The obtained results are as follows:(1) The study of pressure-induced phase transitions and hydrogen-bond insolid N2H4The hydrazine (N2H4) contains as high as12.6wt%of hydrogen and hence it isused as a component with liquid hydrogen in jet fuels because it produces a largeamount of heat when burned. The previous studies on solid hydrazine are mostlyfocused on low-temperature structures, but the high-pressure structure is unclear. Wehave performed the high pressure study of hydrazine by in situ Raman spectroscopyand synchrotron X-ray diffraction experiments up to46.5and33.0GPa, respectively.It is found that the liquid hydrazine solidifies into phase I at about1.2GPa. Thesymmetry of phase I is confirmed to be space group P21by the peak assignment,group theory analysis and Rietveld refinement of XRD patterns. A solid-solidtransition from phase I to II is observed in both Raman spectroscopy and XRDexperiments at about2.4GPa, which is ascribed to the formation of newhydrogen-bonds between hydrazine molecules. At18.4GPa, an isostructuraltransition from phase II to the final phase III is observed. The pressure-inducedadjustment of bifurcated hydrogen-bond is firstly researched and regarded as theorigin of the isostructural transition. Above20.6GPa, a clear softening behavioroccurs in the NH2symmetric stretching mode. The coupling of optical vibrationsderived from enhancement of the hydrogen-bond is proposed as a crucial role in this softening process. This change in Raman spectra is recorded as a typical feature in theprocess of hydrogen-bond symmetrization. By the analysis with DMP theory, it issuggested that the N-H…N hydrogen-bond may symmetries at around60GPa.(2) The high pressure study of solid N2H4H2OThe high pressure behavior of hydrazine monohydrate (N2H4H2O) has beeninvestigated by in situ Raman spectroscopy and synchrotron X-ray diffractionexperiments. It is found that the liquid N2H4H2O solidifies into phase I at3.2GPa.The Raman spectra indicate that the NH3+group forms by the strong attraction to Hcation in phase I. Further solid-solid transition from phase I to II occurs at7.2GPa. Itis attributed to the contortion of N2H4molecules. The first Raman spectralmeasurement performs up to36.0GPa, the spectra show that the OH stretching peaksgradually disappear above20GPa, which is regarded as the typical soft behavior inthe stretching mode during the hydrogen-bond symmetrization. In the process ofcompression, no peak of solid hydrazine and water has been collected. We thusspeculate that the pressure-induced crystal of mixed liquid is pure hydrazinemonohydrate. Upon decompression, the spectra changes a lot at2.3GPa, it issuggested that the sample has resolved. The decomposer is recognized as purehydrazine by comparision of Raman spectra and peak assignment. The second Ramanspectral measurement was performed up to13.3GPa. The result shows that thesample has resolved at around1.9GPa upon decompression, which is agree with thefirst experimental result. The XRD patterns indicate that the sample decomposesabove40.4GPa, which has not changed to1.5GPa upon decompression.(3) The structure and hydrogen-bond study in N2H4HCl under pressureThe first high pressure study of solid hydrazinium monochloride has beenperformed by in situ Raman spectroscopy and synchrotron X-ray diffraction (XRD)experiments in diamond anvil cell (DAC) up to39.5and24.6GPa, respectively. Thestructure of phase I at room temperature is confirmed to be space group C2/c by thePawley refinement of the XRD pattern. The staggered N2H5+ions are connected bythe N-H…N bond in phase I. A structural transition from phase I to II is observed at7.3GPa. The N-H…Cl hydrogen-bond has formed in phase II and causes obvriousFermi resonance between the softing NH stretching mode and lattice anddeformational modes. The pressure shifts of NH stretching peaks are really small,which is attributed to the compensating effects caused of the strong hydrogen-bond to the pressure. Above19.8GPa, the structure further transiforms into phase III. TheFermi resonance disappears completely, indicating that the N-H…Cl hydrogen-bondsymmetrization achieves in phase III. In additation, we observed that the shift of NH2deformational peak show diverse rates in the three phases, which is also the evidencesof phase transitions.(4) The hydrogen-bond symmetrization study in N2H4HBr under pressureThe solid hydrazine monohydrobromide (N2H4HBr) has been first investigatedby in situ Raman spectroscopy, IR and synchrotron X-ray diffraction (XRD)measurements under pressure up to65.4,23.9and27.0GPa, respectively. At ambientconditions, the space group of phase I is confirmed to be C2/c by the peak assignmentof the Raman peaks and Rietveld refinement of XRD patterns. The first solid-solidphase transition from phase I to II is observed at6.6GPa. The Br-ion has formedN-H…Br hydrogen-bond in phase II, and the structure of phase II is confirmed to beP1. At12.7GPa, the structure further transforms into phase III. The obvrious Fermiresonance starts at12.7GPa and completes at24.5GPa. Above34.5GPa, thestructure finally transforms into phase IV with numbers of peaks disappearing in thespectra. It is suggested that the hydrogen-bond symmetrization achieves in phase IV.The phase IV persists up to65.4GPa and the structure returns to phase I at0GPaafter pressure release.更多还原