Title: Structural investigation of three distinct amorphous forms of Ar hydrate

Abstract

Three amorphous forms of Ar hydrate were produced using the crystalline clathrate hydrate Ar·6.5H₂O (structure II, Fd$$\bar{3}$$ with combining macron]m, a ≈ 17.1 Å) as a precursor and structurally characterized by a combination of isotope substitution (³⁶Ar) neutron diffraction and molecular dynamics (MD) simulations. The first form followed from the pressure-induced amorphization of the precursor at 1.5 GPa at 95 K and the second from isobaric annealing at 2 GPa and subsequent cooling back to 95 K. In analogy to amorphous ice, these amorphs are termed high-density amorphous (HDA) and very-high-density amorphous (VHDA), respectively. The third amorph (recovered amorphous, RA) was obtained when recovering VHDA to ambient pressure (at 95 K). The three amorphs have distinctly different structures. In HDA the distinction of the original two crystallographically different Ar guests is maintained as differently dense Ar–water hydration structures, which expresses itself in a split first diffraction peak in the neutron structure factor function. Relaxation of the local water structure during annealing produces a homogeneous hydration environment around Ar, which is accompanied with a densification by about 3%. Upon pressure release the homogeneous amorphous structure undergoes expansion by about 21%. Both VHDA and RA can be considered frozen solutions of immiscible Ar and water in which in average 15 and 11 water molecules, respectively, coordinate Ar out to 4 Å. The local water structures of HDA and VHDA Ar hydrates show some analogy to those of the corresponding amorphous ices, featuring H₂O molecules in 5- and 6-fold coordination with neighboring molecules. However, they are considerably less dense. Most similarity is seen between RA and low density amorphous ice (LDA), which both feature strictly 4-coordinated H₂O networks. It is inferred that, depending on the kind of clathrate structure and occupancy of cages, amorphous states produced from clathrate hydrates display variable local water structures.

Authors:

Brant Carvalho, Paulo H. B.  ^[1];

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Moraes, Pedro Ivo R.  ^[2];

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Leitão, Alexandre A.  ^[2];

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Andersson, Ove  ^[3];

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Tulk, Chris A. ^[4]; Molaison, Jamie ^[4]; Lyubartsev, Alexander P.  ^[1];

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Häussermann, Ulrich  ^[1]

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+ Show Author Affiliations

1. Department of Materials and Environmental Chemistry, Stockholm University, SE-10691 Stockholm, Sweden
2. Department of Chemistry, Federal University of Juiz de Fora, Juiz de Fora, MG, 36036-900, Brazil
3. Department of Physics, Umeå University, Umeå, SE-90187, Sweden
4. Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

Publication Date:

Tue Sep 14 00:00:00 EDT 2021

Research Org.:

Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)

Sponsoring Org.:

USDOE Office of Science (SC); Swedish Foundation for Strategic Research (SSF); Swedish National Graduate School; Swedish Research Council (SRC); Swedish Foundation for International Cooperation in Research and Higher Education (STINT); CAPES

OSTI Identifier:

1820188

Alternate Identifier(s):

OSTI ID: 1903914

Grant/Contract Number:  

AC05-00OR22725; 2018-05973; 88887.304724/2018

Resource Type:

Published Article

Journal Name:

RSC Advances

Additional Journal Information:

Journal Name: RSC Advances Journal Volume: 11 Journal Issue: 49; Journal ID: ISSN 2046-2069

Publisher:

Royal Society of Chemistry (RSC)

Country of Publication:

United Kingdom

Language:

English

Subject:

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY