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Transitions in pressure-amorphized clathrate hydrates akin to those of amorphous ices
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).ORCID iD: 0000-0003-2001-4410
Number of Authors: 42019 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 151, no 1, article id 014502Article in journal (Refereed) Published
Abstract [en]

Type II clathrate hydrates (CHs) were studied by thermal and dielectric measurements. All CHs amorphize, or collapse, on pressurization to 1.3 GPa below 135 K. After heating to 160 K at 1 GPa, the stability of the amorphous states increases in a process similar to the gradual high density to very high density amorphous ice (HDA to VHDA) transition. On a subsequent pressure decrease, the amorphized CHs expand partly irreversibly similar to the gradual VHDA to expanded HDA ice transformation. After further heating at 1 GPa, weak transition features appear near the HDA to low density amorphous ice transition. The results suggest that CH nucleation sites vanish on heating to 160 K at 1 GPa and that a sluggish partial phase-separation process commences on further heating. The collapsed CHs show two glass transitions (GTs), GT1 and GT2. GT1 is weakly pressure-dependent, 12 K GPa(-1), with a relaxation time of 0.3 s at 140 K and 1 GPa; it is associated with a weak heat capacity increase of 3.7 J H2O-mol(-1) K-1 in a 18 K range and an activation energy of only 38 kJ mol(-1) at 1 GPa. The corresponding temperature of GT2 is 159 K at 0.4 GPa with a pressure dependence of 36 K GPa(-1); it shows 5.5 times larger heat capacity increase and 4 times higher activation energy than GT1. GT1 is observed also in HDA and VHDA, whereas GT2 occurs just above the crystallization temperature of expanded HDA and only within its similar to 0.2-0.7 GPa stable pressure range.

Place, publisher, year, edition, pages
2019. Vol. 151, no 1, article id 014502
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Chemical Sciences Physical Sciences
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URN: urn:nbn:se:su:diva-171762DOI: 10.1063/1.5096981ISI: 000474214600007PubMedID: 31272168OAI: oai:DiVA.org:su-171762DiVA, id: diva2:1348479
Available from: 2019-09-04 Created: 2019-09-04 Last updated: 2019-09-04Bibliographically approved

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Carvalho, Paulo H. B. BrantHäussermann, Ulrich
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