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The 27th AIRAPT International Conference on High Pressure Science and Technology
Abstract

Oral


Guest Correlations in Crystalline and Pressure-amorphized Argon Clathrate Hydrate

Authors:
Paulo H. B. Brant Carvalho (MMK SU - Department of Materials and Environmental Chemistry, Stockholm University) ; Ulrich Häussermann (MMK SU - Department of Materials and Environmental Chemistry, Stockholm University) ; Ove Andersson (IF UU - Department of Physics, Umeå University) ; Chris Tulk (ORNL - Chemical and Engineering Materials Division, Oak Ridge National Laboratory) ; Jamie Molaison (ORNL - Chemical and Engineering Materials Division, Oak Ridge National Laboratory)

Abstract:

Clathrate hydrates (CHs) are inclusion compounds consisting of a hydrogen-bonded water framework, which forms cages entrapping guest molecules. The crystalline cubic structure II (CS-II) is important for trapping gases such as O2 and N2, and as a gateway to understanding the empty clathrate structure of ice XVI [1]. CS-II hydrates can form different amorphous phases similarly to pure ice [2-5], and the guest correlations are fundamental to understand this similarity in their crystalline and amorphous forms. However, separating out guest correlations remains a challenge. Here we use neutron scattering to study the CS-II CH in situ and under high-pressure. By synthesizing CHs of NatAr and 36Ar as guests and exploring their crystalline and pressure-amorphized phases under identical conditions, guest correlations could be separated from the host. This approach is only possible because 36Ar scatters neutrons more than 13 times stronger than NatAr. When crystalline CHs, guest correlations were used in understanding the guest local structure and disorder inside cages. On CH polyamorphs, pair-distribution function analysis allowed the identification of short-range guest-host correlations. This provides a conclusive understanding of the structural differences between pressure-amorphized CHs and pure ice, answering the question whether CHs are good proxy systems for ice.

 

 

Figure: (a) In situ neutron powder diffraction patterns of the CS-II argon CH as a function of pressure shows the pressure-induced amorphization happened around 1.5 GPa. 13 times difference in argon correlations on (b) crystalline Bragg diffraction and (c) amorphous neutron structure factor function, S(Q), of the natural abundance and 36-argon isotopes clathrate hydrates.

References

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[2] Amann-Winkel, K., et al., ‘Colloquium: Water’s Controversial Glass Transitions’, Rev. Mod. Phys. 2016, 88.

[3] Andersson, Ove, ‘Glass-Liquid Transition of Water at High Pressure’, Proc. Natl. Acad. Sci. 2011, 108, 11013–11016.

[4] Andersson, Ove, and Yasuhiro Nakazawa, ‘Transitions in Pressure Collapsed Clathrate Hydrates’, J. Phys. Chem. B. 2015, 119, 3846–3853.

[5] Andersson, Ove, and Akira Inaba, ‘Glass Transitions in Pressure-Collapsed Ice Clathrates and Implications for Cold Water’, J. Phys. Chem. Lett. 2012, 3, 1951–1955.