| 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. |
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