Cage occupancies in the high pressure structure H methane hydrate: A neutron diffraction study

C. A. Tulk, D. D. Klug, A. M. Dos Santos, G. Karotis, M. Guthrie, J. J. Molaison, N. Pradhan

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Abstract

A neutron diffraction study was performed on the CD 4: D 2O structure H clathrate hydrate to refine its CD 4 fractional cage occupancies. Samples of ice VII and hexagonal (sH) methane hydrate were produced in a Paris-Edinburgh press and in situ neutron diffraction data collected. The data were analyzed with the Rietveld method and yielded average cage occupancies of 3.1 CD 4 molecules in the large 20-hedron (5 126 8) cages of the hydrate unit cell. Each of the pentagonal dodecahedron (5 12) and 12-hedron (4 35 66 3) cages in the sH unit cell are occupied with on average 0.89 and 0.90 CD 4 molecules, respectively. This experiment avoided the co-formation of Ice VI and sH hydrate, this mixture is more difficult to analyze due to the proclivity of ice VI to form highly textured crystals, and overlapping Bragg peaks of the two phases. These results provide essential information for the refinement of intermolecular potential parameters for the water-methane hydrophobic interaction in clathrate hydrates and related dense structures.

Original languageEnglish
Article number054502
JournalJournal of Chemical Physics
Volume136
Issue number5
DOIs
StatePublished - Feb 7 2012

Funding

This work was funded by the Oak Ridge National Laboratory (ORNL), managed by UT-Battelle, LLC for the U. S. Department of Energy (DOE) under Contract No. DE-AC05-00OR22725. Experiments were supported by Spallation Neutron Source and Center of Nanophase Materials Science of ORNL. M.G. and G.K. were funded as part of the EFree, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001057.

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