Structural And Dynamical Properties Of Hydrogen Sulfide And Ammonia Borane Under High Pressure: An Ab Initio Molecular Dynamics Study

Hydrogen-containing molecules are of fundamental importance for their widely existence in nature. The structural and dynamical properties of hydrogen-containing molecules under high pressure can provide basic understanding to interior earth and planetary physics, because high-pressure can greatly modify the structures and chemical bonds including hydrogen bonding which is one of the most important interactions in nature.One of the most interesting phenomena in the hydrogen-containing molecules is the molecular dissociation which turns the system from molecular to atomic at high pressure. For ice, dissociation coincides with hydrogen bond symmetrization, forming ice X, in which the proton occupies the midpoint between two neighboring O atoms. Path integral and ab initio molecular dynamics showed that the proton behavior in ice VIII, VII, and X plays a crucial role in understanding the nature of dissociation. For hydrogen sulfide, which is an analog of water at molecular level, the dissociation turns out to be an intermediate process of the decomposition. Experimental works based on analyzing the decomposed products cannot give a microscopy description to this process and thus failed to provide dissociation mechanism. In this study, by using ab initio molecular dynamics, the stable structure of phase IV was found along with a new-found structure of phase V. The proton behavior in phase V is the key to understanding the decomposition process of hydrogen sulfide.We first chose an experimental proposal for the structure of phase IV (with symmetry of I41/acd) by XRD experiments as our starting structure. We found that the I41/acd structure has very high potential energy. The hydrogen bonding network was reformed in less than 300 fs in a simulation at 15 GPa and 100 K. A structure with a space-symmetry of Ibca and a stable hydrogen bonding network was reached. We found that the hydrogen bonding geometry in the Ibca structure is common in many ice phases and is more energy favorable during geometry optimization. The calculated enthalpy of the Ibca structure is 262 meV/molecule lower than that of I41/acd structure at 15 GPa. Moreover, the Ibca structure was also proposed for an intermediate phase, named phase IV'by Fujihisa et al. In the latter work, the authors found that in the range of 4-10 GPa and 30-250 K, the strongest peak of the X-ray diffraction patterns is doublet, while singlet in phase IV. In our Ibca model, we find that whether the strongest peak is singlet or doublet depends on the b/a ratio. When this ratio is very close to one, the 202 and 022 planes have very similar d-spacing, leading the strongest X-ray peak to appear as a singlet above 12 GPa. However, our calculations suggest that the space group is still the orthorhombic Ibca due to positions of hydrogen atoms also when the Bravais lattice turns tetragonal (a=b). Therefore we propose that phase VI'and phase IV are actually the same phase. The Ibca structure was then heated up to 400 K with a temperature interval of 50 K at 15 GPa by a series of simulations. The radial distribution functions (RDF) and two self-defined angle correlation functions (ACF) were calculated simultaneously to monitor the structural change during the heating. We found that at 350 K, the cell shape changed significantly and the lattice parameters stabilized to new values in about 2 ps. In the S-S RDF, the shoulder of the first main peak disappears above 350 K, indicating a change in the S-S distance distribution. In the S-H RDF, the sharp separation between the first and second peaks which represent the S-H covalent bond and the S…H hydrogen bond, respectively, disappears at 350 K. This points to the occurrence of occasional hopping of protons between neighboring molecules. The calculated ACF indicate that molecular orientations change rapidly above 350 K and the structures become orientationally disordered. To obtain this disordered structure, we first calculated the averaged sulfur lattice which turns out to have a space-symmetry of P63/mmc. Next, the density distribution of H atoms from our MD trajectory was extracted by translating H atoms to the P63/mmc unit cell with periodic conditions. It turns out that there are 12 density maxima of hydrogen around each sulfur atom which correspond to the Wyckoff positions of space group P63/mmc. This suggests that our model is a proton-disordered structure with fractional occupation of hydrogen positions. The calculated X-ray diffraction curves consist with the experimental XRD patterns very well. We believe that our obtained P63/mmc structure is the experimental suggested unreacted crystalline structure of phase V. The behavior of the protons in equilibrated phase IV and V is expatiated by the calculated distribution of proton positions along the S-H…S bond. In phase IV, the protons are well localized, meaning that the hydrogen sulfide molecules are well defined as stable units and the structure of phase IV is ordered. While in phase V, both the rotational disorder and thermal jumps of protons exist. The protons in phase V are delocalized and can move back and forth along the S-S separation. The proton behavior in phase V provides a possible way to the decomposition of hydrogen sulfide, i.e., when the S-S covalent bonds are formed, protons can be expelled from the sulfur lattice by a combination of molecular rotation and proton jumping.The importance of hydrogen-containing molecules also originates from their application in hydrogen storage materials. One of the most promising candidates is ammonia borane (NH3BH3, AB) which contains remarkable high gravimetric and volumetric hydrogen density, and has a moderate dehydrogenation temperature. Experimental attempts were focused on the use of acid, transition metal catalysts, ionic liquids, nano-scaffolds, and etc. to improve the efficiency of discharging H2 from AB. The structural and dynamical factors that govern the stability and the intermolecular interactions are thus essential in understanding and improving the rate of hydrogen releasing. The structures, phase transitions, and hydrogen dynamics of AB at ambient pressure were studied extensively by former researchers. It turns out that AB has two ambient-pressure solid phases with Pmn21 symmetry and I4mm symmetry, respectively. The Pmn21-Cmc21 phase transition is proved to be to be a first-order phase transition leading by progressive displacement of the borane group under the amine group. Using different techniques based on nuclear magnetic resonance (NMR), it is suggested that the rotational dynamics of the NH3 and BH3 groups in the ambient-pressure phases plays a crucial role in the phase transition and shows distinct rotational barriers compared with gas state. The separation of the NH3 and BH3 rotation in the ambient-pressure phases is also supported by both theoretic study and neutron scattering experiments and is attributed to the different inter- and intramolecular torsional forces. The dihydrogen bond, which is formed between the protonic and hydridic hydrogen atoms at adjacent NH3 and BH3 ends respectively plays a significant role in determining the properties of solid AB. The existence of dihydrogen bonds in AB is also supported by high-pressure Raman studies, in which a red shift in the N-H stretch frequency and a blue shift in the B-H stretch frequency with increasing pressure are observed. According to these Raman spectroscopic studies, AB shows a complex vibrational behavior at high pressure and room temperature, and has at least three phase transitions up to 20 GPa at room temperature. However, a recent X-ray diffraction study proposed that only one phase transition occurred at ~1.1 GPa up to 12.1 GPa at room temperature. To the best of our knowledge, no theoretical studies were focused on the phase transitions at high pressure. To filling in this gap, ab initio molecular dynamics were performed extensively in this study. Another interest in this study is the hydrogen dynamics at high pressure, including proton distribution and rotation of the NH3 and BH3 groups. The dihydrogen bond which plays a significant role in determining the behavior of AB is discussed.Gradually applying pressures at 100 K, the Pmn21 structure transformed to the Cmc21 structure at 3 GPa. This indicates that the Cmc21 phase observed by a recent room-temperature X-ray diffraction study can also be obtained at low temperature and at high pressure. The phase transition is found to be preceded by displacement of the NH3 and BH3 groups in opposite directions, resulting new dihydrogen bonding arrangement. Next, a series of simulations at raising pressure and at room temperature were performed up to 60 GPa in order to search new high-pressure phases. We found that the Cmc21 structure is stable until the pressure reaches 15 GPa, at which the supercell as and bs exhibit alternately up-and-down undulating. As the pressure reaches 20 GPa, the supercell lattice as and bs converged to different values within 2 ps. A new structure with monoclinic P21 symmetry emerged, denoted as P21(Z=2) for there are 2 AB molecules in the unit cell. We also found that the deformation of the supercell at 15 GPa and 300 K is corresponding to a shear strain of the Cmc21 unit cell. The calculated elastic constants indicate that there is a softening of C66 at 15.3 GPa and zero-temperature which is in consistent with the MD simulations. In considering the uncertainties due to the finite simulation time and the temperature effect, we propose that AB undergoes a mechanical instability induced phase transition at 15 GPa and 300 K, and this transition corresponds to the transition at 12 GPa suggested by recent Raman spectrum study. A further phase transition took place at 50 GPa and 300 K, a new structure with P21 symmetry and 4 AB molecules in its unit cell emerged (denoted as P21(Z=4)). In the P21(Z=4) phase, the staggered conformation of AB molecules was broken, resulting in different dihydrogen bonding geometry. We also calculated the enthalpy difference curves which support our MD results. A structural analyse suggests that the dihydrogen bonds per molecule in the Cmc21 structure increase with rising pressure. We use an empirical rule for the dihydrogen bonding geometry as a criterion. It turns out that the number of dihydrogen bonds increases from 12 to 14 at 6.4 GPa, which is in contrast to the proposal by Y. Filinchuk et al. The calculated N-H-H-B dihedral angle distribution derived from MD trajectories indicates that the AB molecules no longer stay in their stable molecular conformation in the P21(Z=4) phase. Further, the calculated dihydrogen bond angles suggest that the NHH angle is more linear and the BHH angle is more bent in all high-pressure phases, which is in agreement with the empirical rule established by W. Klooster et al. The NHH angle change little under compression, while the BHH angles first increase with rising pressure, and become almost as linear as the NHH angle in the P21(Z=2) phase. However, in the P21(Z=4) phase, by breaking the staggered molecular conformation, the BHH angle decreases significantly which is due to the reorientation of dihydrogen bonding network. Our MD simulations also indicate that the rotational angles of the NH3 and BH3 groups represent layered distribution in all high-pressure phases at room temperature. The angle interval is approximate 120°which does not break the symmetry of the high pressure phases due to the C3v point symmetry of AB molecules. As a result, all the three-pressure phases remain ordered structures. We also found that the separation of the NH3 and BH3 rotation observed in the ambient-pressure phases is remained in the high-pressure phases. Moreover, we calculated the potential energy surfaces for rotation of the NH3 and BH3 groups by rotating the NH3 and BH3 groups recurrently over the range of 0≤.θNH3≤120°and 0≤.θBH3≤120°in 10°increments with respect to the equilibrious configurations. Thus, the energy barriers for rotation of the NH3 and BH3 groups can be estimated from theθBH3=0 path and theθNH3=0 path, respectively. As for the correlated rotation of the whole molecule, it is theθBH3=θNH3 path. It is found that the rotational barriers of NH3 is much small than those of BH3 in all high-pressure phases, which is in agreement with the observation in the MD that the NH3 groups rotate more frequently than BH3.