Infrared absorption spectra for solid ammonia were measured with diamond anvil cell at room temperature and pressures up to 120 GPa. Two phase transitions were observed at about 40 and 70 GPa, transition from solid V to VI and to solid VII. Four branch peaks were observed for the NH stretching vibrations. One branch showed a significant frequency decrease from 3200 cm-1 at 10 GPa to 3000 cm-1 at 120 GPa, while the other two branches markedly showed the peak shifts to high frequency. No evidence was obtained for the hydrogen-bond symmetrization. [phase transition, hydrogen-bond symmetrization, solid ammonia, infrared spectroscopy, high pressure]
High-pressure X-ray and neutron diffraction analyses of an ambient-pressure phase (AP) and two high-pressure phases (HP1 and HP2) of ammonia borane (i.e., NH3BH3 and ND3BD3) were conducted to investigate the relationship between their crystal structures and dihydrogen bonds. It was confirmed that the hydrogen atoms in AP formed dihydrogen bonds between adjacent molecules, and the H–H distance between the hydrogen atoms forming this interaction was shorter than 2.4 Å, which was nearly 2 times larger than the van der Waals radius of hydrogen. In the case of half of the hydrogen bonds, a phase transition from AP to the first high-pressure phase (HP1) at ∼1.2 GPa resulted in an increase in the H–H distances, which suggested that the dihydrogen bonds were broken. However, when HP1 was further pressurized to ∼4 GPa, all of the H–H distances became shorter than 2.4 Å again, which implied the occurrence of pressure-induced re-formation of the dihydrogen bonds. It was speculated that the re-formation was consistent with a second-order phase transition suggested in previous studies by Raman spectroscopy and X-ray diffraction measurement. Furthermore, at ∼11 GPa, HP1 transformed to the second high-pressure phase (HP2), and its structure was determined to be P21 (Z = 2). In this phase transition, the inclination of the molecule axis became larger, and the number of types of dihydrogen bonds increased from 6 to 11. At 18.9 GPa, which was close to the upper pressure limit of HP2, the shortest dihydrogen bond decreased to ∼1.65 Å. Additionally, the X-ray diffraction results suggested another phase transition to the third high-pressure phase (HP3) at ∼20 GPa. The outcomes of this study confirmed experimentally for the first time that the structural change under pressure causes the breakage and re-formation of the dihydrogen bonds of NH3BH3.
An X-ray diffraction measurement and a density functional theory (DFT) calculation of the pressure-induced transformation in LiBH4 were performed. The structure of its first high pressure phase at room temperature (phase-III) was determined to be I4 1 /acd structure, whose unit cell was a 2 × 2 ×2 supercell of Ama2 structure proposed previously. Pressure-induced transformation from phase-III took place at 16 GPa and the structure of the second high-pressure phase was analyzed as I4/mmm structure (phase-V′ ), in which hydrogen atoms were disordered. However, an annealing treatment for phase-V′under high-pressure suggested that it was a metastable phase. With the pressure elevated up to 30 GPa, the tetragonal I4/mmm structure was gradually transformed to a cubic Fm˜3m structure, which has been reported as a stable phase of phase-V previously. The P T diagram was examined using high-pressure/high-temperature Raman scattering, and pressure/temperature dependence of the relative ionic conductivity was observed across the phase boundaries.
The crystal structure and phase transition of cubic structure II (sII) binary clathrate hydrates of methane (CH 4 ) and propanol are reported from powder X-ray diffraction measurements.T he deformation of host water cages at the cubic-tetragonal phase transition of 2-propanol + CH 4 hydrate,b ut not 1-propanol + CH 4 hydrate,w as observed belowabout 110 K. It is shown that the deformation of the host water cages of 2-propanol + CH 4 hydrate can be explained by the restriction of the motion of 2-propanol within the 5 12 6 4 host water cages.T his result provides al ow-temperature structure due to atemperature-induced symmetry-lowering transition of clathrate hydrate.This is the first example of acubic structure of the common clathrate hydrate families at af ixedc omposition.Clathrate hydrates,a lso known as gas hydrates,a re hostguest compounds that are crystalline materials consisting of water molecule frameworks.The host water cages of clathrate hydrate are hydrogen-bonded and encage guest molecules inside via van der Waals interactions. [1] Each guest molecule is encaged into different types of cages ordered in threedimensional structures according to the size of the guest molecule.T hree different crystal structure of clathrate hydrate are well known depending on type of guest molecules;c ubic structure I( sI) with space group Pm3 n,c ubic structure II (sII) with space group Fd3 m, and hexagonal structure H( sH) with space group P6/mmm. [2] These host structures are analogues of the semiconducting Group 14 clathrates [3] and silica clathrates (so-called clathrasils). [4] Natural-gas or methane hydrate is now seen as apossible global source of methane, [5,6] and the capability of gas hydrates to store large amounts of gas has opened up possibilities for potential industrial applications such as hydrogen [7,8] and ozone [9,10] storage.P hysical properties of gas hydrates are attributed to host-guest interactions such as rattling motions [11] or hydrogen bonding [12,13] of guest mole-cules.H ost-guest interactions also play ac rucial role,a sg as hydrate structures are thermodynamically stable only when am inimum number of cages are occupied by the guest molecules,depending on their nature.Phase changes of clathrate hydrates are known to take place with pressure,and cubic clathrate structures,sIand sII, at ambient pressure transform to hexagonal sH or astructure closely related to it at pressure above 0.5 GPa. [14] Up to now, however, few examples of the phase change with temperature have been reported in the common clathrate hydrate structural families sI, sII, and sH at constant composition. [15,16] On the other hand, clathrasils have av ariety of polyhedral cages and show temperature-induced phase changes. [17] Given the analogy between oxygen linking tetrahedral atoms in silicate framework structure in clathrasils and hydrogen bonds linking oxygen atoms in clathrate hydrates, [18,19] there is ag reat interest in determining whether lower symmetry clathrate hydrates are indeed realized from cubic clathrate st...
LiBH4·H2O, obtained by exposing LiBH4 to air with about 5% relative humidity at room temperature is characterized by powder XRD, IR transmission spectroscopy, and DFT calculations.
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