Symmetrization and Chemical Precompression Effect of Hydrogen-Bonds in H2-H2O System
doi: 10.11858/gywlxb.20190730
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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。Abstract: Hydrogen hydrate (H2-H2O) excited significant interest as an environmentally clean and efficient hydrogen storage material. Here we conducted a high-pressure experimental research on hydrogen hydrate combined with in-situ Raman spectroscopy and synchrotron X-ray diffraction measurements. Our results indicated that the cubic C2 phase with stoichiometry 1∶1 of H2 and H2O transformed to a new tetragonal phase C3 after packing more hydrogen molecules above 24.5 GPa. The structure of C3 was determined to be P41 with a 1∶2 ratio of H2O to H2, and could survive down to 8.6 GPa upon decompression. Two districted behaviors of guest hydrogen clusters were observed with increasing pressure. One showed blue-red frequency shift transition similarly as pure hydrogen, the other continuously blue-shifted to higher frequencies in the whole pressure range. Fermi resonance between the deformational mode and softened stretching mode was firstly detected, indicating that the hydrogen-bond was symmetrized at around 55 GPa. The complicated behaviors of hydrogen molecules and interactions with water molecules in hydrogen hydrate provided a different insight into the guest-host system under pressure.
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Key words:
- high pressure /
- hydrogen-bond symmetrization /
- hydrogen hydrate
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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].
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