Symmetrization and Chemical Precompression Effect of Hydrogen-Bonds in H2-H2O System

JIANG Shuqing YANG Xue WANG Yu ZHANG Xiao CHENG Peng

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Symmetrization and Chemical Precompression Effect of Hydrogen-Bonds in H2-H2O System

    作者简介: JIANG Shuqing (1987-), Ph.D, major in high-energy material synthesis under extreme high-temperature and high-pressure conditions. E-mail: jiangshuqing@issp.ac.cn;

Symmetrization and Chemical Precompression Effect of Hydrogen-Bonds in H2-H2O System

    Author Bio: JIANG Shuqing (1987-), Ph.D, major in high-energy material synthesis under extreme high-temperature and high-pressure conditions. E-mail: jiangshuqing@issp.ac.cn;
  • 摘要: 氢水化合物作为潜在的环境友好型储氢含能材料引起了众多关注。结合金刚石对顶砧装置和原位拉曼光谱测量、同步辐射X射线衍射光谱测量两种表征手段,试图深入理解高压驱动下氢的特征行为,寻找可能的高压富氢相。结果显示,目前已知最高含氢比例1∶1的相C2在压力24.5 GPa时发生相变,更多的氢分子特征峰随相变出现。通过对理论预测结构的拟合,该相最终被确定为P41,氢水比例达到2∶1,且在卸压时能够稳定保存至8.6 GPa。考虑到冰中氢键对称化对压致相变和结构稳定性的重要作用,着重观测了氢键的行为,首次探测到水分子之间氢键对称化过程中完整的费米共振现象。通过对O-H对称伸缩振动模式软化行为的拟合,最终确定氢键对称化发生在55 GPa,同时拉曼光谱测量显示有更进一步相变伴随发生。氢水化合物中不同氢团簇对化学预压作用表现出截然不同的应激反应,这在此体系中也是首次被注意到,对含氢体系和纯氢中氢的金属化研究具有一定参考作用。
  • Figure 1.  The evolution of Raman spectra of hydrogen hydrate with increasing pressure from 0.4 to 71.4 GPa. (a) The lattice peaks in the region of 0–1200 cm–1, where the spectra in each phase were marked by different colors and divided by dashed lines; the arrows marked three new broad peaks in high pressure phase IV; (b) The stretching peaks of H2 at various pressures, symbol * referred a new peak at 25.6 GPa in phase III, and the two shoulders at 55.5 GPa were marked by black arrow.

    Figure 2.  The frequency shifts of H-H stretching mode in hydrogen hydrate at various pressures in this work and referred researches. Symbols: the solid black and blue circles indicated frequency shifts collected in this work; the red triangle was frequency shift of pure H2 while the rest black triangle, green square and blue circle were hydrogen in hydrogen hydrate which were cited from previous experimental and theorical studies[15, 25, 30]. The pressure boundaries of each phase were marked by vertical dashed lines.

    Figure 3.  The synchrotron XRD measurement of hydrogen hydrate up to 63.0 GPa. The phase C2 was confirmed by characteristic diffraction lines (111), (220), (311), (400), (331) and (422) at 7.6 GPa. The splitting of line (220) was marked by red arrows at 24.5 GPa, while the splitting of line (111) at 48.9 GPa was marked by black arrows. A rough Pawley refinement of the theorical predicted P41 structure was performed with our experimental spectrum of 24.5 GPa, the short blue lines showed the calculated diffraction lines of refined structure with Materials Studio program.

    Figure 4.  (a) The soften evolution of O-H stretching peaks with increasing pressure from 2.2 to 29.0 GPa; the intensity of deformational peak increased above 29.0 GPa owing to the Fermi resonance with the soften O-H stretching mode, which further shifted to low frequency region above 49.0 GPa. (b) The pressure dependence of the O-H stretching mode; the blue line guided the extrapolation of the soften O-H stretching mode, the red dashed line showed the O-H stretching mode in solid water for comparison[31].

    Figure 5.  (a) The Raman spectra of hydrogen hydrate under decompression. (b) The comparison of pressure dependences of the H-H stretching mode during the compression and decompression process. The red and black arrows marked the disappeared peaks at 39.7 and 8.6 GPa, respectively.

  • [1] WIGNER E, HUNTINGTON H B. On the possibility of a metallic modification of hydrogen [J]. The Journal of Chemical Physics, 1935, 3(12): 764–770. doi: 10.1063/1.1749590
    [2] ASHCROFT N W. Metallic hydrogen: a high-temperature superconductor? [J]. Physical Review Letters, 1968, 21: 1748–1749. doi: 10.1103/PhysRevLett.21.1748
    [3] BARBEE T W III, GARIA A, COHEN M. First-principles prediction of high-temperature superconductivity in metallic hydrogen [J]. Nature, 1989, 340: 369–371. doi: 10.1038/340369a0
    [4] CUDAZZO P, PROFETA G, SANNA A, et al. Ab initio description of high-temperature superconductivity in dense molecular hydrogen [J]. Physical Review Letters, 2008, 100(25): 257001. doi: 10.1103/PhysRevLett.100.257001
    [5] HUESTIS D L. Hydrogen collisions in planetary atmospheres, ionospheres, and magnetospheres [J]. Planetary and Space Science, 2008, 56(13): 1733–1743. doi: 10.1016/j.pss.2008.07.012
    [6] NELLIS W J. Unusual magnetic fields of uranus and neptune: metallic fluid hydrogen [J]. Modern Physics Letters B, 2015, 29(1): 1430018. doi: 10.1142/S021798491430018X
    [7] ASHCROFT N W. Hydrogen dominant metallic alloys: high temperature superconductors? [J]. Physical Review Letters, 2004, 92(18): 187002. doi: 10.1103/PhysRevLett.92.187002
    [8] MOHTADI R, ORIMO S. The renaissance of hydrides as energy materials [J]. Nature Reviews Materials, 2017, 2(3): 16091. doi: 10.1038/natrevmats.2016.91
    [9] VOS W L, FINGER L W, HEMLEY R J, et al. Novel H2-H2O clathrates at high pressures [J]. Physical Review Letters, 1993, 71(19): 3150. doi: 10.1103/PhysRevLett.71.3150
    [10] VOS W L, FINGER L W, HEMLEY R J, et al. Pressure dependence of hydrogen bonding in a novel H2O-H2 clathrate [J]. Chemical Physics Letters, 1996, 257: 524–530. doi: 10.1016/0009-2614(96)00583-0
    [11] MAO W L, MAO H K, GONCHAROV A F, et al. Hydrogen clusters in clathrate hydrate [J]. Science, 2002, 297: 2247–2249. doi: 10.1126/science.1075394
    [12] PATCHKOVSKII S, JOHN S T. Thermodynamic stability of hydrogen clathrates [J]. Proceedings of the National Academy of Sciences, 2003, 100(25): 14645–14650. doi: 10.1073/pnas.2430913100
    [13] MAO W L, MAO H K. Hydrogen storage in molecular compounds [J]. Proceedings of the National Academy of Sciences, 2004, 101(3): 708–710. doi: 10.1073/pnas.0307449100
    [14] LOKSHIN K A, ZHAO Y S, HE D W, et al. Structure and dynamics of hydrogen molecules in the novel clathrate hydrate by high pressure neutron diffraction [J]. Physical Review Letters, 2004, 93(12): 125503. doi: 10.1103/PhysRevLett.93.125503
    [15] MACHIDA S, HIRAI H, KAWAMURA T, et al. Structural changes of filled ice Ic structure for hydrogen hydrate under high pressure [J]. The Journal of Chemical Physics, 2008, 129(22): 224505. doi: 10.1063/1.3013440
    [16] MACHIDA S, HIRAI H, KAWAMURA T, et al. Structural changes and intermolecular interactions of filled ice Ic structure for hydrogen hydrate under high pressure [J]. Journal of Physics: Conference Series, 2010, 215: 012060. doi: 10.1088/1742-6596/215/1/012060
    [17] STROBEL T A, HESTER K C, KOH C A, et al. Properties of the clathrates of hydrogen and developments in their applicability for hydrogen storage [J]. Chemical Physics Letters, 2009, 478: 97–109. doi: 10.1016/j.cplett.2009.07.030
    [18] MACHIDA S, HIRAI H, KAWAMURA T, et al. Raman spectra for hydrogen hydrate under high pressure: intermolecular interactions in filled ice Ic structure [J]. Journal of Physics and Chemistry of Solids, 2010, 71: 1324–1328. doi: 10.1016/j.jpcs.2010.05.015
    [19] MACHIDA S, HIRAI H, KAWAMURA T, et al. Isotopic effect and amorphization of deuterated hydrogen hydrate under high pressure [J]. Physical Review B, 2011, 83: 144101. doi: 10.1103/PhysRevB.83.144101
    [20] STROBEL T A, SOMAYAZULU M, HEMLEY R J. Phase behavior of H2+H2O at high pressures and low temperatures [J]. The Journal of Physical Chemistry C, 2011, 115: 4898–4903. doi: 10.1021/jp1122536
    [21] BORSTAD G M, YOO C S. H2O and D2 mixtures under pressure: spectroscopy and proton exchange kinetics [J]. The Journal of Chemical Physics, 2011, 135(17): 174508. doi: 10.1063/1.3658485
    [22] EFIMCHENKOA V S, KUZOVNIKOVA M A, FEDOTOVA V K, et al. New phase in the water-hydrogen system [J]. Journal of Alloys and Compounds, 2011, 509(Suppl 2): S860–S863.
    [23] ZHANG J Y, KUO J L, IITAKA T. First principles molecular dynamics study of filled ice hydrogen hydrate [J]. The Journal of Chemical Physics, 2012, 137(8): 084505. doi: 10.1063/1.4746776
    [24] HIRAI H, KAGAWA S, TANAKA T, et al. Structural changes of filled ice Ic hydrogen hydrate under low temperatures and high pressures from 5 to 50 GPa [J]. The Journal of Chemical Physics, 2012, 137(7): 074505. doi: 10.1063/1.4746017
    [25] QIAN G R, LYAKHOV A O, ZHU Q, et al. Novel hydrogen hydrate structures under pressure [J]. Scientific Reports, 2014, 4: 5606.
    [26] SAUNDERS S R J, MONTEIRO M, RIZZO F. The oxidation behaviour of metals and alloys at high temperatures in atmospheres containing water vapour: a review [J]. Progress in Materials Science, 2008, 53(5): 775–837. doi: 10.1016/j.pmatsci.2007.11.001
    [27] MOSHARY F, CHEN N H, SILVERA I F. Pressure dependence of the vibron in H2, HD, and D2: implications for inter-and intramolecular forces [J]. Physical Review B, 1993, 48(17): 12613–12619. doi: 10.1103/PhysRevB.48.12613
    [28] DUAN D, LIU Y, TIAN F, et al. Pressure-induced metallization of dense (H2S)2 H2 with high-Tc superconductivity [J]. Scientific Reports, 2014, 4: 6968.
    [29] GONCHAROV A F, LOBANOV S S, PRAKAPENKA V B, et al. Stable high-pressure phases in the H-S system determined by chemically reacting hydrogen and sulfur [J]. Physical Review B, 2017, 95(14): 140101. doi: 10.1103/PhysRevB.95.140101
    [30] CHEN J, GONCHAROV A F, SHUKLA V, et al. Stability of Ar (H2)2 to 358 GPa [J]. Proceedings of the National Academy of Sciences, 2017, 114(14): 3596–3600. doi: 10.1073/pnas.1700049114
    [31] GONCHAROV A F, STRUZHKIN V V, MAO H K, et al. Raman spectroscopy of dense H2O and the transition to symmetric hydrogen bonds [J]. Physical Review Letters, 1999, 83(10): 1998–2001. doi: 10.1103/PhysRevLett.83.1998
    [32] PRUZAN P, WOLANIN E, GAUTHIER M, et al. Raman scattering and X-ray diffraction of ice in the megabar range: occurrence of a symmetric disordered solid above 62 GPa [J]. The Journal of Physical Chemistry B, 1997, 101(32): 6230–6233. doi: 10.1021/jp963182l
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  • 收稿日期:  2019-02-26
  • 录用日期:  2019-03-21
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  • 刊出日期:  2019-04-01

Symmetrization and Chemical Precompression Effect of Hydrogen-Bonds in H2-H2O System

    作者简介:JIANG Shuqing (1987-), Ph.D, major in high-energy material synthesis under extreme high-temperature and high-pressure conditions. E-mail: jiangshuqing@issp.ac.cn
  • 1. Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, 230031 Hefei, China
  • 2. University of Science and Technology of China, Hefei 230026, China

摘要: 氢水化合物作为潜在的环境友好型储氢含能材料引起了众多关注。结合金刚石对顶砧装置和原位拉曼光谱测量、同步辐射X射线衍射光谱测量两种表征手段,试图深入理解高压驱动下氢的特征行为,寻找可能的高压富氢相。结果显示,目前已知最高含氢比例1∶1的相C2在压力24.5 GPa时发生相变,更多的氢分子特征峰随相变出现。通过对理论预测结构的拟合,该相最终被确定为P41,氢水比例达到2∶1,且在卸压时能够稳定保存至8.6 GPa。考虑到冰中氢键对称化对压致相变和结构稳定性的重要作用,着重观测了氢键的行为,首次探测到水分子之间氢键对称化过程中完整的费米共振现象。通过对O-H对称伸缩振动模式软化行为的拟合,最终确定氢键对称化发生在55 GPa,同时拉曼光谱测量显示有更进一步相变伴随发生。氢水化合物中不同氢团簇对化学预压作用表现出截然不同的应激反应,这在此体系中也是首次被注意到,对含氢体系和纯氢中氢的金属化研究具有一定参考作用。

English Abstract

  • Hydrogen is the most widespread element in the universe with lots of unique properties and potential applications. The metallic hydrogen was not only predicted to be a high Tc superconductor at extreme pressures, but also to be a high energy density material, which would have great applications in military projects and energy sources[14]. The pressure-induced insulator to metal transition of hydrogen are of fundamental interest to condensed matter physics and chemistry as well as planetary astronomy[56]. In view of the difficulties in technology of pressing pure hydrogen, the metallization pressure of hydrogen is proposed to decrease in the hydrogen-rich materials because of the chemical precompression effect[7]. Many hydrogen-rich compounds are also high energetic, or potential hydrogen storage materials, attracting considerable attentions in recent years[8].

    Hydrogen hydrate was known as the typical clathrate hydrates in solid state at high pressures, exciting great interest for its unusual behaviors under pressure and expected hydrogen storage properties. A series of researches on solid hydrogen hydrate were performed at various pressures and temperatures by experiments and theoretical simulations[925]. Two typical clathrate structures, filled ice II and Ic (phase C1 and C2) at relatively low pressures were firstly defined in 1993[9]. The guest hydrogen molecules occupied the voids in the host water framework with a very high stoichiometry of 1∶1 in phase C2, which was attractive as a hydrogen-storage material. The subsequent works suggested the cubic phase would survive up to 60 GPa as long as the structural stability enhanced with hydrogen-bond symmetrization above 30 GPa[10] . However, Machida et. al[16, 19] found that hydrogen molecules extracted from the C2 phase at around 20 GPa, giving rise to a transition to poor-hydrogen phase. The new phase was predicted as the trigonal or tetragonal symmetries in X-ray diffraction (XRD) measurements and molecular dynamics simulation[15, 2324]. Moreover, the ab initio variable-composition evolutionary simulations showed that the C2 phase would lose stability and transformed to a tetragonal phase with 2∶1 hydrogen and water molecules[25] .

    In hydrogen hydrate, the stability of clathrate structure was mainly depended on the framework of host water molecules, which was assembled by hydrogen-bonds. Therefore, the pressure of hydrogen-bond symmetrization would be significantly lower than that in pure ice because of the packing efficiency of the guest hydrogen molecules[10]. The Fermi resonance should also be observed as a convincing evidence of hydrogen-bond symmetrization, while it has not been detected up to now[15] . Furthermore, the behaviors of hydrogen molecules and hydrogen-bonds were expected to show a different insight of precompression effect in this typical of material with hydrogen contained, because it had been proved that the interaction between hydrogen and water molecules played a significant role in phase transitions[18]. We expected that a systematic experimental study combined with diamond anvil cell (DAC) and in-situ spectroscopy measurements would give a clear understanding on the phase transitions, hydrogen behaviors and hydrogen-bond symmetrization in the hydrogen hydrate system.

    Here, the hydrogen hydrate was studied by the in-situ Raman spectroscopy and synchrotron XRD measurements up to 71.4 and 63.0 GPa, respectively. Two distinct pressure-induced behaviors of hydrogen clusters were observed above 24.5 GPa. The Fermi resonance appeared at 29.0 GPa and persisted up to 49.0 GPa. The extrapolation line indicated that the hydrogen-bonds in the water framework symmetrized at around 55 GPa in hydrogen hydrate.

    • Symmetric DACs equipped with flat anvils of $ 300\;{\text{μ}}{\rm{m}}$ was used to generate pressures in in situ Raman scattering and angle-dispersive XRD measurements. Rhenium gasket (with an initial thickness of $ 200\;{\text{μ}}{\rm{m}}$) was preindented to about $ 30\;{\text{μ}}{\rm{m}}$ and then the sample cavity was drilled in the centre with a diameter of $ 120\;{\text{μ}}{\rm{m}}$. The pressure in the DAC was calibrated by the ruby fluorescence method. A tiny aluminum ball with a diameter of $ 10-15\;{\text{μ}}{\rm{m}}$ and liquid deionized water were loaded in the chamber. The hydrogen was generated after an excess laser heating run on the aluminum ball at 0.4 GPa. The final products in the chamber were hydrogen and aluminum oxide[26]. The CW solid state laser at 1064 nm was introduced into the optical system using polarizing beam-splitter cubes and focused into a flat top focal spot from two sides of the DAC using the Mitutoyo near-IR×20 and ×10 long-working distance objective lenses. The focal laser spots were about $ 10-15\;{\text{μ}}{\rm{m}}$ in diameter, making it possible to heat the aluminum balls. The backscattering geometry was adopted for confocal measurements with incident laser wavelengths of 457, 488, and 561 nm in the Raman measurements (the 488 nm laser line was used in the current measurement). The XRD experiments were collected at the synchrotron beam line, sector 13 of the Advanced Phonon Source (APS) of the Argonne National Laboratory with the wavelengths of 0.3344 Å (GSECARS).

    • The pure hydrogen hydrate sample was generated after sufficient laser heating runs at 0.4 GPa before further compression. As shown in Fig.1, the representative vibrons for hydrogen rotons and stretching mode appeared at 358, 590, 816, 1035 and 4146 cm–1. With further compression to 2.2 GPa, the liquid sample crystallized to phase I with appearance of several sharp Raman peaks. The phase I was described as clathrates phase C1 in the first study[9]. In the clathrates structure, the hydrogen molecules occupied the voids of the hydrogen-bonded water framework, it was therefore recognized as a filled ice phase with space group $ R\bar 3$ defined by XRD measurement on a single crystal. The other clathrates phase (C2) was also observed at 3.4 GPa in this experiment as shown in Fig.1(a), whose structure was confirmed as Fd3m by XRD in previous study[9]. The stoichiometry of hydrogen and water molecules was almost 1∶1 in phase C2, which therefore attracted many attentions as a potential hydrogen-storage material.

      Figure 1.  The evolution of Raman spectra of hydrogen hydrate with increasing pressure from 0.4 to 71.4 GPa. (a) The lattice peaks in the region of 0–1200 cm–1, where the spectra in each phase were marked by different colors and divided by dashed lines; the arrows marked three new broad peaks in high pressure phase IV; (b) The stretching peaks of H2 at various pressures, symbol * referred a new peak at 25.6 GPa in phase III, and the two shoulders at 55.5 GPa were marked by black arrow.

      With increasing pressure, the rotation mode of H2 at 375 cm–1 splitted into two peaks while other peaks in the region of 0–1200 cm–1 became much broader above 23.0 GPa. A new stretching peak of hydrogen with frequency of 4929 cm–1 appeared at 25.6 GPa. Therefore, it is inferred that a solid-solid transition from C2 to a new phase occurred at 23.0 GPa, which was named as C3 by convention in the hydrogen hydrate system. The pressure dependences of the stretching peaks were plotted in Fig.2. It is notable that the stretching peak at 4929 cm–1 blue-shifted to 4254 cm–1 below 38.4 GPa at first, and then red-shifted to 4231 cm–1 persistently up to 71.4 GPa. The frequency transition from blue-shift to red-shift was similar to the characteristic behavior of pure hydrogen under pressure[27] . Therefore, it was simply attributed to the extraction of guest hydrogen molecules at the ultimate pressure in previous experimental studies[16, 19]. However, similar transformation was also observed in the S-H system during the synthesis of (H2S)2H2 from H2S, and (H2S)2H2 became a currently known highest Tc superconductor in its Im-3m phase[2829]. Meanwhile, the ab initio variable-composition evolutionary simulation predicted that the C2 phase would become metastable and transformed to a novel tetragonal phase C3 (P41) with 2∶1 ratio of hydrogen and water molecules[25]. In the predicted phase C3, eight hydrogen molecules were located at the center of chair-like H-O rings therefore it had the highest hydrogen concentration among all hydrogen hydrates. A further XRD measurement was performed up to 63 GPa here to explore the structure of C3. As shown in Fig.3, the phase C2 was determined by its characteristic diffraction lines of (110), (220), (311), (400), (331) and (422) at 7.6 GPa[9]. Above 24.5 GPa, a splitting of (220) line was observed, which was marked by red arrows in Fig.3, indicating the structure transformed from cubic Fd3m to tetragonal space[24]. A rough Pawley refinement of the theorical predicted P41 structure was performed with our experimental spectrum of 24.5 GPa. The refined lattice parameters were a = b = 5.046 Å and c = 5.523 Å, in consistent with the simulated results[25].

      Figure 2.  The frequency shifts of H-H stretching mode in hydrogen hydrate at various pressures in this work and referred researches. Symbols: the solid black and blue circles indicated frequency shifts collected in this work; the red triangle was frequency shift of pure H2 while the rest black triangle, green square and blue circle were hydrogen in hydrogen hydrate which were cited from previous experimental and theorical studies[15, 25, 30]. The pressure boundaries of each phase were marked by vertical dashed lines.

      Figure 3.  The synchrotron XRD measurement of hydrogen hydrate up to 63.0 GPa. The phase C2 was confirmed by characteristic diffraction lines (111), (220), (311), (400), (331) and (422) at 7.6 GPa. The splitting of line (220) was marked by red arrows at 24.5 GPa, while the splitting of line (111) at 48.9 GPa was marked by black arrows. A rough Pawley refinement of the theorical predicted P41 structure was performed with our experimental spectrum of 24.5 GPa, the short blue lines showed the calculated diffraction lines of refined structure with Materials Studio program.

      In the filled ice structures, the water molecules were connected by O-H···O hydrogen-bonds and further formed clathrate framework after crystallization. As the H proton occupied the midpoint between two oxygen ions, the O-H···O hydrogen-bonds became symmetric. In solid water, the phase X with symmetric hydrogen-bond formed at 60 GPa and kept stable to at least 210 GPa[31]. It was reported that the correlation between the νO-H and dO-H···O in hydrogen hydrate was similar as solid water, making the clathrate a model system for ice at pressures reduced by a factor 2 because of the packing efficiency in the filled ice structures[910]. Therefore, it is considered that this symmetric bonding construction is more important to the stability of high-pressure phases in hydrogen hydrate. As shown in Fig.4, the O-H stretching peaks moved into the diamond 2rd Raman vibration region and disappeared at 29.0 GPa. Meanwhile, a weak peak appeared at 1549 cm–1 with intensity increasing gradually. It was assigned to the typical vibrational coupling between the deformational mode and soften stretching mode, which was the characteristic behavior during hydrogen-bond symmetrization and also had been detected at about 45 GPa in solid ice[31]. The coupling effect enhanced with pressure increasing up to 40 GPa and then disappeared above 45.8 GPa, indicating that the soften stretching mode further shifted into lower frequency region. Above 53.0 GPa, it is notable that three broad peaks at 539, 783 and 856 cm–1 became more intense with increasing pressure, which were attributed to coupling between the soften stretching mode and the translational and rotational modes. We further simulated the frequency shifts of the soften stretching mode by extrapolation of the existed data with the function νOH=(ν02aP)1/2 given in Ref.[32], where ν0 and a are parameters, suggesting a soft-mode behavior. As shown in Fig.4(b), the transition region of 52–57 GPa was in well consistent with our Raman spectra as shown in Fig.1(a), suggesting that the hydrogen-bond symmetrized at around 55 GPa.

      Figure 4.  (a) The soften evolution of O-H stretching peaks with increasing pressure from 2.2 to 29.0 GPa; the intensity of deformational peak increased above 29.0 GPa owing to the Fermi resonance with the soften O-H stretching mode, which further shifted to low frequency region above 49.0 GPa. (b) The pressure dependence of the O-H stretching mode; the blue line guided the extrapolation of the soften O-H stretching mode, the red dashed line showed the O-H stretching mode in solid water for comparison[31].

      With increasing pressure, two weak shoulders marked by arrows in Fig.1(b) appeared at 4539 and 4599 cm–1 at 55.5 GPa, indicating that the hydrogen-bond symmetrization completed at around 55 GPa and caused further phase transition from phase C3 (space group P41) to a new high-pressure phase IV (named C4 as customary). Meanwhile, three broad peaks marked by arrows appeared throughout the region of 400–1000 cm–1, which were assigned to the complicated coupling between the soften stretching modes and the translational and rotational modes[31]. Moreover, only one splitting of line (111) which was related to hydrogen atoms of water molecule was collected in the XRD patterns at 48.9 GPa, indicating that the phase IV was slightly distorted Phase C3 as in the case of phase VII of ice[31].

      The interactive effect between guest hydrogen and host water molecules played a vital role in stabilizing the filled ice structure. Basically, it lowered the pressure of hydrogen-bond symmetrization as described above. Furthermore, it can provide extra positive pressure acting as the chemical precompression effect on the guest hydrogen molecules. As shown in Fig.2, the stretching peak of guest hydrogen blue-shifted from 4136 cm–1 at 0.4 GPa to 4535 cm–1 at 71.4 GPa, which had not drawn sufficient attentions in previous studies. In the most recent study, similar abnormal blue-shift behavior of hydrogen vibron was interpreted as a negative chemical pressure on H2 molecules in Ar(H2)2[30]. Obviously, the enhancement of blue-shift was attributed to a larger chemical pressure coming from packing efficiency. Although the interpretation of a negative chemical effect was in consistent with the metallization mechanism of molecule dissociation, however, here we preferred to assign this behavior to the isolated guest hydrogen clusters in the specific filled ice structures, which was differ from the bulk pure hydrogen sample.

      During decompression, the phase transitions were reversible with great hysteresis as shown in Fig. 5. The phase C4 persisted down to 39.7 GPa with disappearance of shoulders marked by red arrows, while the tetragonal hydrogen rich phase C3 even survived to 8.6 GPa with the disappearance of representative stretching peak marked by black arrows. Moreover, it is notable that the stretching peak at higher frequency showed consistent shifting tendency in the whole experimental circle, indicating that the pressure induced behaviors of guest hydrogen molecules were relatively independent to the phase transitions in hydrogen hydrate system.

      Figure 5.  (a) The Raman spectra of hydrogen hydrate under decompression. (b) The comparison of pressure dependences of the H-H stretching mode during the compression and decompression process. The red and black arrows marked the disappeared peaks at 39.7 and 8.6 GPa, respectively.

    • Raman spectroscopy and synchrotron XRD measurements of hydrogen hydrate were performed at high pressures. A new tetragonal hydrogen rich phase C3 was identified above 24.5 GPa by experiments in consistent with previous theorical simulation. The Fermi resonance of the deformational mode and the soften stretching mode was firstly observed in Raman spectroscopy between 29.0–45.8 GPa, providing strong evidence of hydrogen-bond symmetrization at around 55 GPa. Two distinct pressure-induced behaviors of guest hydrogen molecules were observed in the sample, presenting complicated interactions among guest hydrogen molecules, host water molecules and external pressures. These behaviors would give a further understanding of the guest-host system.

      Acknowledgements: We thank Alexander F. Goncharov for his assistance in the experiment.

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