Phase changes of filled ice Ih methane hydrate under low temperature and high pressure
Phase changes of filled ice Ih methane hydrate under low temperature and high pressure
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DOI：
10.1063/1.4820358
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发表时间：
2013-09-14
影响因子：
4.4
通讯作者：
Irifune, Tetsuo
中科院分区：
文献类型：
--
作者：
Tanaka, Takehiko;Hirai, Hisako;Irifune, Tetsuo
在2.0 ~ 77.0 GPa和30 ~ 300 K条件下，利用金刚石砧细胞和氦冷冻恒温器对甲烷水合物进行了充冰Ih结构的低温高压实验。原位x射线衍射揭示了宿主骨架轴向比的压缩性随压力的明显变化。拉曼光谱显示了客体甲烷分子的C-H振动模式的分裂，这在之前被解释为客体分子的取向顺序。振动模态分裂时的压力和温度条件与压缩率变化时的压力和温度条件吻合较好。结果表明：(1)客体甲烷分子在高压低温条件下由定向无序状态发生定向有序；（ii）这种客体排序导致宿主框架的各向异性收缩。这种客体定向排序和随后宿主框架的各向异性收缩与先前报道的填充冰Ic氢水合物相似。由于将客人点餐方式不同的相位视为不同的相位，因此通过x射线研究粗略估计客人无序相和客人有序相的存在区域。此外，在客序相的压力之上，在低温区域又形成了另一个高压相。研究了氘化水主体样品，观察了同位素效应对客体有序和相变的影响。(C) 2013 AIP出版有限责任公司
Low-temperature and high-pressure experiments were performed with filled ice Ih structure of methane hydrate under 2.0-77.0 GPa and 30-300 K using diamond anvil cells and a helium-refrigeration cryostat. In situ X-ray diffractometry revealed distinct changes in the compressibility of the axial ratios of the host framework with pressure. Raman spectroscopy showed a split in the C-H vibration modes of the guest methane molecules, which was previously explained by the orientational ordering of the guest molecules. The pressure and temperature conditions at the split of the vibration modes agreed well with those of the compressibility change. The results indicate the following: (i) the orientational ordering of the guest methane molecules from an orientationally disordered state occurred at high pressures and low temperatures; and (ii) this guest ordering led to anisotropic contraction in the host framework. Such guest orientational ordering and subsequent anisotropic contraction of the host framework were similar to that reported previously for filled ice Ic hydrogen hydrate. Since phases with different guest-ordering manners were regarded as different phases, existing regions of the guest disordered-phase and the guest ordered-phase were roughly estimated by the X-ray study. In addition, above the pressure of the guest-ordered phase, another high-pressure phase developed in the low-temperature region. The deuterated-water host samples were also examined, and the influence of isotopic effects on guest ordering and phase transformation was observed. (C) 2013 AIP Publishing LLC.