The structure of CO ₂ hydrate between 0.7 and 1.0 GPa

Journal Article · Fri Nov 07 00:00:00 EST 2014 · Journal of Chemical Physics

DOI:https://doi.org/10.1063/1.4899265· OSTI ID:1387495

Tulk, C. A.; Machida, S.; Klug, D. D.; Lu, H.; Guthrie, M.; Molaison, J. J.

A deuterated sample of CO2 structure I (sI) clathrate hydrate (CO2·8.3 D2O) has been formed and neutron diffraction experiments up to 1.0 GPa at 240 K were performed. The sI CO2 hydrate transformed at 0.7 GPa into the high pressure phase that had been observed previously by Hirai et al. [J. Phys. Chem. 133, 124511 (2010)] and Bollengier et al. [Geochim. Cosmochim. Acta 119, 322 (2013)], but which had not been structurally identified. The current neutron diffraction data were successfully fitted to a filled ice structure with CO2 molecules filling the water channels. This CO2+water system has also been investigated using classical molecular dynamics and density functional ab initio methods to provide additional characterization of the high pressure structure. Both models indicate the water network adapts a MH-III “like” filled ice structure with considerable disorder of the orientations of the CO2 molecule. Furthermore, the disorder appears to be a direct result of the level of proton disorder in the water network. In contrast to the conclusions of Bollengier et al., our neutron diffraction data show that the filled ice phase can be recovered to ambient pressure (0.1 MPa) at 96 K, and recrystallization to sI hydrate occurs upon subsequent heating to 150 K, possibly by first forming low density amorphous ice. Unlike other clathrate hydrate systems, which transform from the sI or sII structure to the hexagonal structure (sH) then to the filled ice structure, CO2 hydrate transforms directly from the sI form to the filled ice structure.

• Research Organization: Energy Frontier Research Centers (EFRC) (United States). Energy Frontier Research in Extreme Environments (EFree)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES)
• DOE Contract Number: SC0001057
• OSTI ID: 1387495
• Journal Information: Journal of Chemical Physics, Vol. 141, Issue 17; Related Information: EFree partners with Carnegie Institution of Washington (lead); California Institute of Technology; Colorado School of Mines; Cornell University; Lehigh University; Pennsylvania State University; ISSN 0021-9606
• Publisher: American Institute of Physics (AIP)
• Country of Publication: United States
• Language: English

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