Studies Of Typical Hydrogen-rich Compounds And The Related System Under High Pressure

Hydrogen is the first element in the periodic table of elements, whichalso has the simplest atomic structure in all elements. The research ofhydrogen not only has academic significance, but also is crucial inapplication. On one hand, the hydrogen gas is considered as one kind ofalternative clean energy in the future, especially used in automobile as fuel,but in the process of hydrogen energy promotion, the hydrogen storage inautomobile is still unsolved. So the hydrogen-rich compounds havebecome a hot issue. The previous theoretical and experimental studieshave found that the new high pressure phases with higher hydrogencontent probably exhibited better hydrogen storage properties, so theresearch on hydrogen-rich compounds under high pressure is of greatsignificance. In recent years, the research of hydrogen-rich compoundsunder high pressure have attracted much attention, including thetransformation sequence, high pressure structures and phase transitionmechanism, the insulator-metal transition and improving the superconductor temperature. We have selected alkli metal nitrogenhydride in the ternary hydrides and ZrH₂in the binary hydrides as themain objects of this study. On the other hand, hydrogen will become metalhydrogen under high pressure, and metal hydrogen is one of the mostimportant physical problems. As a potential room-temperaturesuperconductor, metal hydrogen has not been obtained in experiment.However, the dissociation process and mechanism of hydrogen under highpressure is critical, so we choose the related molecular system （SnBr4） tostudy its behavior under high pressure for obtaining the useful physicalpatterns and laws. In this article, we are going to study hydrogen-richcompounds and molecular system by in situ experimental measurementsand first principle calculations. And the obtained results are as follows:（1） Study of alkali metal nitrogen hydride under high pressure. Inthe alkali metal nitrogen hydride system, we focus on LiNH₂and NaNH₂.In this system, some methods are being used in order to make it easier forthe hydrogen released, including adding catalyst or ball milling method.High pressure is one kind of effective method to find new materials, soalkali metal nitrogen hydrides have been investigated under high pressurefor obtaining more hydrogen storage materials with excellentperformances. The previous high pressure studies have explored thehydrides using the Raman spetra measurements. However, Raman spectracan not determine the crystal structure of the new phases. In this work, wehave studied the structural stability of LiNH₂under high pressure bymeans of synchrotron radiation XRD and first-principles calculation, andthe maximum pressure is28.0GPa. It is found that the sample changesfrom a tetragonal phase （space group I-4） to the monoclinic phase （spacegroup P21） in the range of10.3–15.0GPa. In this transformation process,the volume collapse is about11%, which is larger than many othercomplex hydride ones. In addition, through the calculation of the density of state of the two phases, we have revealed the reasons of the largevolume collapse. By in situ Raman spectrum measurement andsynchrotron radiation XRD study of NaNH₂, two structural changes werefound at about1.12GPa and1.93GPa, respectively, and it is proposedthat that the two crystal structures of the two new phases belong tomonoclinic and orthorhombic system, respectively.（2） Effect of hydrostatic conditions on high-pressure behavior ofZrH₂. In order to improve the performance of hydrogen storage, a fullunderstanding of its behaviors under pressure is considered as essential.Moreover, the application of high pressure is an effective way to explorethe hydrogen-metal interactions which are of great relevance to specialproperties as well as hydrogen storage applications. As for the transitionmetal dihydrides, although many literatures have reported their groundstate properties and electronic properties by theoretical calculations,high-pressure experiments have not been intensively carried out. As one ofthe most important hydrogen storage material, the ZrH₂sample has beenmeasured by in situ synchrotron XRD under hydrostatic andnonhydrostatic compressions, respectively. By comparing the latticeconstants as a function of pressure under these two compressions, twodistinct compression regimes can be identified around22.0GPa in thehydrostatic compression while the lattice constants are linearly dependenton high pressure. So we have observed that ZrH₂has unusual compressionbehaviors under hydrostatic pressure, but the tetragonal structure of ZrH₂is stable up to43.8GPa under nonhydrostatic compression. Further, thevolume reduction under hydrostatic pressure is nearly equal with that ofnonhydrostatic compression below22.0GPa, above which the volumereduction starts to lower than that of nonhydrostatic pressure. Weexplained the remarkable phenomena that the sample underwent anisostructural phase transition under hydrostatic pressure. （3） Structural and electronic changes of SnBr4under highpressure. As important molecular crystals, group IV halides have recentlybeen investigated for understanding structural changes and amorphousmechanism. Although several models have been proposed, the mechanismof pressure-induced amorphization observed in group IV halides is stillunder debate. As one of the group IV halides, previous high pressurestudies are only limited to low pressure and Spectral measurement. In thispaper, we have explored the pressure induced amorphization of SnBr4bysynchrotron X-ray diffraction, Raman spectra measurements, opticalabsorption/transmission measurements and ab initio calculations. Ourresult shows that there is a crystal-crystal transition induced by themolecular dimerization at about10.8GPa, which completes at about21.5GPa. Combining the Raman spectroscopy results with theoreticalcalculations, it is found that the crystal-crystal transition happens due tothe molecular dimerization. And the crystal structure of the dimeric phaseis obtained by the theoretical calculations with space group P-1. Thegradual amorphization is found to transform into non-molecularamorphous phase above34.7GPa, and the amorphization process does notcomplete until43.8GPa. The joint of experimental and theoretical resultsshows that the SnBr4molecular crystal becomes the non-molecular phaseupon compression. The band gap of the sample deceases with increasingpressure （from3.4eV at ambient pressure to1.5eV at21.0GPa）. Ourexperimentally optical transmission spectra indicate a possibleinsulator-metal transition at about40.3GPa.