Studies Of Typical Hydrogen-rich Compounds Under High Pressure

Hydrogen is the first element in the periodic table，and it also has thesimplest atomic structure in all elements. Scientific research workerslaunched a wide range of research on it because of the uniqueness of thehydrogen. So far, they study the academic significance and applicationvalue of hydrogen mainly from two aspects. On one hand,it is well knownthat hydrogen is likely become a kind of non-pollution clean energy in thenear future, and hydrogen can be used as fuel in many areas. However, inthe process of achieve this goal, hydrogen storage or loading has not beenperfectly solved. So, find a new way of hydrogen storage became theresearch focus. Therefore, the study of hydrogen-rich compoundsbecomes very important. From the existing study, both in experimentsand theory found that under high pressure hydrogen-rich compounds arelikely to have better performance of hydrogen storage. So the mechanismand properties of the hydrogen-rich compound at high pressure, isparticularly important. In addition, the atmospheric hydrogen is likely tobe transformed into the metal under high pressure, and become aroom-temperature superconductor. But so far it has not found metalhydrogen in experiments, given compress directly to gas hydrogen needexactly high pressure, so that we try to study hydrogen-rich compoundsunder high pressure, in order to reduce the pressure of metallization, and provide the necessary guidance to the metallization of hydrogen. In thiswork, we selected the typical ammonia borane and halogenatedammonium hydrogen-rich compounds as the main research object, tostudy the behavior and the mechanism of such system under highpressure, mainly used in situ high-pressure Raman scattering andsynchrotron radiation XRD experiments, combined with the firstprinciples calculations, and get the following innovative results:(1)Experimental verification of the high pressure crystalstructures in NH3BH3and the dihydrogen bonding network underhigh pressure. As a promising candidate material for hydrogen storage,ammonia borane (NH3BH3) has attracted significant interest. It has a highgravimetric (19.6wt.%) and volumetric (145gH/L) hydrogen density.While, thermal decomposition of ammonia borane proceeds in three stepsand the release of all the hydrogen atoms is only accomplished at500C.Although temperature plays an important role in controlling the stabilityof materials, as an independent thermodynamic parameter, pressure cansignificantly alter the crystal structures and properties of materials. Atambient conditions, ammonia borane crystallizes ina tetragonal spacegroup I4mm (α-NH3BH3) with a unit cell containing two molecules.Thisphase transform to orthorhombic Cmc21structure (β-NH3BH3) at2.14GPa. However, there is no consensus about the behavior of NH3BH3under higher pressure. In order to identify the crystal structure of the newphase, we have considered two experimental structural models (P1andP21(Z=4)) and one theoretical model (P21(Z=2)) to perform theRietveld refinement. The observed XRD pattern was well refined with theproposed space group P21containing two molecules in a unit cell(γ-NH3BH3). Fitting the measured volumetric compression data to thethird order Birch Murnaghan equation of state reveals a bulk modulus of B0=9.9±0.5and17.0±3.0GPa (with fixed B0′=4) for the β-NH3BH3below and above5GPa, respectively. Still, with the splitting of the NBHrock mode in Raman experiment, it is concluded that a second-orderisostructural phase transition occurs at5GPa. By analyzing thedihydrogen bonding framework, the origin of the isostructural phasetransition is attributed to the number of dihydrogen bondings permolecule in the Cmc21phase increasing from12to14at5GPa.(2)Pressure-induced structural changes in NH4Br and thechange of hydrogen bond under high pressure. Since the discovery ofa sharp anomaly in the specific heat of ammonium bromide (NH4Br) atabout-30°C by Simon, which is accompanied by a volume anomaly inthe same crystal structure above and below the region of anomaly,ammonium halides have attracted much experimental and theoreticalinterest. Hydrostatic compression also decreases volume, so we haveexplored the effect of pressure to complement previous work at lowtemperatures. As an independent thermodynamic parameter, pressure cansignificantly alter the crystal structures and properties of materials. Thestrong influence of pressure on phase transitions between these structuresin NH4Br has already been established by different methods. However,the complete structural information of phase V has not been detected byXRD method so far. Despite the orientations of the tetrahedral NH+4ionscause the structural transitions, the hydrogen bond plays an important rolein the phase transitions in NH4Br. We report angle dispersive X-raydiffraction measurements and Raman spectroscopy on NH4Br up to70.0GPa at room temperature. Three thermodynamically stable phases (phaseII, IV and V) are confirmed and a new phase (phase VI) of P21/msymmetry is proposed by the Rietveld Refinement methodfor the firsttime. The phase sequence observed in NH4Br is in accordance with the phase IIâ†'IVâ†'Vâ†'VI. The phase V transforms into the phase VI at about57.8GPa with a huge volume reduction of30%. Still, the intramoleculardistances are measured to better understand the nature of structures. TheH-H interactions become markedly more important as the N-Br distancesare compacted, which is probably the reason of the kink of symmetricstretching band (ν1) at the transition pressure.(3)Structural properties of ammonium iodide under highpressure. The behaviors of ammonium iodide (NH4I) under high pressureare of great interest in recent years. Ammonium ions in NH4I areorientationally disordered in the phases I and II, which exhibit cubic NaCland CsCl-type structures, respectively. As the pressure increases to about27kbar, phase II transforms to phase IV, which also has the CsC1-typestructure but exhibits a parallel ordering of ammonium ions. As thepressure increases further up to54kbar, the phase transition to a highpressure phase V occurs. At room temperature the following sequence ofphase transitions isexpected in NH4I (ND4I): I-II-IV–V. Thetransitions between the known phases have been spectroscopicallyexamined quite frequently. However, the structural information of phaseV has not been detected by X-ray measurements yet.The application ofpressure make it change in sign of the hydrogen bond in NH4Br becauseof the H–H repulsion. As such, to a big surprise, will the change in NH4Ibe similar with NH4Br? The high-pressure behaviors of ammoniumiodide (NH4I) have been investigated by in situ synchrotron X-raydiffraction (XRD) and Raman scattering up to40GPa.The first-orderphase transition from phase IV to V isconfirmed by XRD measurementsfor the first time. Fitting the measured volumetric compression data to thethird order Birch-Murnaghan equation of state reveals a bulk modulus ofB0=14±1and28±2GPa (with fixed B0′=4) for the phase IV and V, respectively. Still, by analyzing the red shift of N-H symmetric andasymmetric mode and the intramolecular distances, it is concluded thathydrogen bond is the dominant effect upon compression during the wholepressure range.