High-pressure study on symmetric hydrogen bond in gas hydrates

在甲烷水合物的“填充icg”相，理论上估计在40 GPa下发生氢键对称化，比冰VII的压力低20 GPa。然而，迄今为止，还没有实验证据表明甲烷水合物存在对称氢键，因为没有人观察到甲烷水合物“填充冰”相的O-H伸缩振动峰。因此，本研究以甲烷水合物“填充冰”单晶体的生长为目的，利用高压拉曼光谱技术研究甲烷水合物“填充冰”相氢键的对称性，并采用可控温度的高压金刚石对顶砧装置成功地生长出甲烷水合物“填充冰”小单晶体。通过对上述单晶体的高压拉曼光谱分析，我们最终观察到了10 GPa以上的“填充冰”相的O-H伸缩振动。虽然我们没有直接观察到对称氢键，但从10 ~ 15 GPa压力范围内O-H伸缩模的拉曼频移随压力的变化，我们发现甲烷水合物中氢键对称化应该发生在50 GPa左右。

In the "filled icg" phase of methane hydrate, it is theoretically estimated that the pressure-induced symmetrization of hydrogen bond occurs at 40 GPa which is 20 GPa lower than that of ice-VII. However, until now there is no experimental evidence for symmetric hydrogen bond of methane hydrate, because no one observed the O-H stretching vibrational peak in "filled ice" phase of methane hydrate. Therefore, the purposes of this research are to grow a single crystalline "filled ice" of methane hydrate and to explore the symmetrization of hydrogen bond in "filled ice" phase of methane hydrate using high-pressure Raman spectroscopy.We succeeded in growing small single crystals of "filled ice" of methane hydrate using a temperature controlled high-pressure diamond anvil cell. By applying the high-pressure Raman spectroscopy to the above single crystals, we finally observed the O-H stretching vibration in "filled ice" phase above 10 GPa. Although we did not still observed the symmetric hydrogen bond directly, we found the possibility that the symmetrization of hydrogen bond in methane hydrate should occur at about 50 GPa from the pressure dependence of Raman frequency shifts for O-H stretching mode at pressures from 10 to 15 GPa.