本文基于第一性原理计算,对三种不同结构的Ⅰ型甲烷水合物进行弛豫,得到优化后的结构、电子态密度及光学性质,通过对结果的分析揭示不同笼子占据率对水合物的结构和相关性质的影响。这三种结构分别为:(cI) 只有一个大笼未被占据; (cII)只有一个小笼未被占据; (cIII) 每个笼子都被甲烷分子占据。结果显示,cIII是最稳定的,因为它有完美的晶体结构;cII和cI相对较不稳定,其中cI在缺失一个甲烷分子的情况下,结构出现了较大的变形,这导致它成为三种情况中最不稳定的结构;相反,同样缺失了一个甲烷分子的cII的结构变形就很小。cII和cI的体积变化分别是0.56%和2.1%,cII的电子态密度和能带几乎与cIII的一致,而cI的则与前两者差别很大。计算结果表明,相对于电子转移,质子无序对水合物的介电常数的影响更为主要;甲烷水合物只对紫外光有响应,无论结构和占据率差异如何,甲烷水合物都具有相近的光学性质。本文的结果证明,缺失一个甲烷分子,对cII造成的影响很小,其晶体结构、介电常数和光学性质与cIII的一致,然而同样缺失一个甲烷分子的cI的结构及性质却出现了巨大变化。这些结果可为水合物探测提供有价值的参考。
Abstract
To reveal the effects of different cage occupations on hydrate structure and related properties. The relaxation of the structure, electronic density of state and optical properties of the SI methane hydrates with three different configurations are calculated by first-principle, including (cI) only one large cage lacks its unique methane molecules; (cII), only one of the small cage lacks its unique methane molecule; (cIII) each cage is fully filled with methane molecules. The results show that the hydrate is most stable in the cIII due to its perfect structure, the cII is less stable, whereas the cI is least stable owing to its larger structural distortion after the loss of methane. On the contrary, loss of a methane molecule from cII causes negligible deformation. The relative change in cell volume is 0.56% and 2.1%, corresponding to the cII and the cI, respectively. The electronic density of states and the energy band gap of the cII is almost the same with those of cIII, differing obviously with those of cI. The calculation results display that the contribution of electronic transition is small, and proton disorder is the dominant hydrate permittivity. Methane hydrate is only responsive to light in the ultraviolet region, revealing their similar properties, regardless of their structural discrepancies, or/and the different ratios of water and methane molecules, 46/8=5.75 versus 46/7=6.57. Our calculations demonstrate that the lack of one methane in the cII causes negligible influence to the lattice structure and therefore to the electronic and optical properties in comparison with the cIII, whereas the lack of one methane in the cI can cause detectable changes. These results might provide valuable reference to the industrial exploration.
关键词
甲烷水合物 /
占据率 /
光学性质 /
第一性原理
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Key words
methane hydrate /
cage occupancy /
optical properties /
the first-principle
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脚注
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基金
海南省重点研发计划项目(ZDYF2017098);浙江省自然科学基金项目(LY18E010007)
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