Raman spectroscopy of chibaite, natural MTN silica clathrate, at high pressure up to 8 GPa

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Highlights

  • Monotonic versus irregular behavior of hydrocarbon and lattice bands.

  • Different high-pressure behavior of methane hosted in large and small cages.

  • Two reversible transitions in chibaite are revealed at 1.2 and 3.5 GPa.

Abstract

The high-pressure behavior of chibaite, newly discovered natural clathrasil isotypical to gas hydrate sII, with ideal formula 136SiO2·24X (X = CH4, C2H6, C3H8, C4H10), was studied by Raman spectroscopy up to 7.7 GPa with a diamond anvil cell, using KBr as a pressure transmitting medium. The chibaite structure is preserved throughout the whole pressure range. The deviation from regular shift and appearance of new bands in the lattice spectrum is observed at about 1.2 and 3.5 GPa. As concerns the guest hydrocarbon molecules, the high-frequency shifts of their C–C and C–H bands are almost monotonic within the whole pressure range, showing only slight bends at 1.2 and 3.5 GPa. The FWHMs of stretching C–C and C–H bands demonstrate a high sensitivity to pressure, more pronounced at 1.2 GPa. The obtained data allow to suggest two reversible structural transformations in chibaite at about 1.2 and 3.5 GPa, associated with the distortion of framework cages and corresponding ordering of the guest hydrocarbon molecules. The role of ‘guest molecule/host cage’ dimensional ratio determining the HP behavior of MEP and MTN clathrasils is discussed.

Introduction

Silica clathrate compounds (or clathrasils) present an important group of microporous materials composed of pure SiO2 framework, in which cages are filled with different molecules [1]. Despite a large variety of synthetic compounds, only one natural clathrasil, rare mineral melanophlogite, was known so far [2]. The framework of melanophlogite (MEP-type), isotypic to cubic gas hydrate sI [2], [3], is built up of corner-linked SiO4-tetrahedra to form two types of cages, [512] and [51262], where the superscripts refer to the number of pentagonal or hexagonal faces. In these cages, gaseous phases such that CH4, CO2 and N2 can occur, amounting up to 8–10 wt% of bulk composition [4], [5]. Another natural clathrasil, mineral chibaite, isotypical with cubic gas hydrate sII, was recently discovered to occur in marine sediments [6]. Unlike melanophlogite, its framework topology (MTN-type) is based on the ‘dodecasil layer’ formed by face-sharing [512] pentagonal dodecahedra cages; such layers are stacked in ABC sequence and form additional large [51264] cages (Fig. 1). The presence of dodecasil layers determines an appreciably higher portion of small [512] cages in chibaite structure, as compared to melanophlogite, which is also seen from the ideal formulas of melanophlogite (46SiO2·2 M12·6 M14) and chibaite (136SiO2·16 M12·8 M16), where M is the guest molecule in the 12, 14 or 16-coordinated cage. The guest molecular compounds in chibaite are presented, in addition to CH4, by heavier hydrocarbons such as C2H6, C3H8, i-C4H10 [6]. From these, only methane can enter small [512] cages (5.7 Å in diameter), whereas larger hydrocarbons can be located only in the large [51264] cages (7.5 Å in diameter) [7]. It is interesting that, in isostructural gas hydrates, heavy hydrocarbons also tend to concentrate in sII type structure, being more stable as compared to sI structure.

These naturally occurring gas-bearing clathrasils are extremely interesting because they give evidence for the participation of microporous silicates in hydrocarbons transport within the Earth's crust, which is also connected to the gas storage problem. It is therefore important to characterize their high-pressure (HP) behavior with the emphasis on the state of the guest molecules and their possible role in the structure evolution, as well as on pressure stability limits of these compounds.

The MEP and MTN clathrasils are known to be stable up to about 6–8 GPa; at that, their rigid frameworks experience only minor structural transformations. In melanophlogite, a cubic-to-tetragonal phase transition with a ≈0.1% volume effect was observed at P > 1.14 GPa [8]. It is interesting that, in synthetic MTN clathrasil, no change in the diffraction profile was observed with respect to drastic changes in the framework vibrations at 4.5 GPa [9]. The regular contraction, without volume discontinuities, was also predicted for empty MEP clathrasil by molecular dynamics calculations [10]. Despite the non-ambient behavior of these two compounds has many common features, it appears to depend on the guest composition: for example, a retransition to cubic phase after the temperature decrease is observed only in MTN clathrasil filled with pyrolidine, but not in the case of enclathrated t-butilamin [11]. This emphasizes again a well established structure-directing role of the guest compounds in clathrasils [12], [13] and makes important the comparison of the non-ambient behavior of different enclathrated species within the same framework.

From this point of view, chibaite presents an interesting example of MTN clathrasil filled with smallest organic molecules (predominantly CH4). On the other hand, it seems important to compare the high-pressure behavior of chibaite with melanophlogite, having the same abundant guest species but slightly different framework.

Raman spectroscopy is one of the most convenient methods to characterize the state of enclathrated molecules [14], [15]. At ambient conditions, similarly to isostructural gas hydrates, the vibrational frequencies of the guest molecules in melanophlogite and chibaite only slightly differ from those in the free state, which implies orientational disordering and weak dispersion interactions of the guest molecules with the silica framework [5], [6]. In sI and sII gas hydrates, a different location of CH4 molecules in small and large cages is established from the splitting of the intense C–H stretching band of methane into two components at 2912 and 2905 cm−1, respectively [15], [16]. Such assignment is supported by different HP behavior of these two bands: the high-frequency component, corresponding to methane in small cage, shifts appreciably with pressure, whereas the low-frequency component retains its wavenumber almost constant [17]. A weak dependence on pressure is considered to reflect a very loose cage environment for a small CH4 molecule, despite a pressure-induced contraction of the cage. The pressure shift towards higher wavenumber is commonly observed for methane in small cages and for heavier hydrocarbons in all types of cages in gas hydrates [17], [18].

In contrast to extensive HP spectroscopic studies of enclathrated hydrocarbon molecules in gas hydrates [17], [18], their HP behavior was not characterized neither for the MEP nor MTN-type clathrasils. In the present work we use Raman spectroscopy to characterize the HP behavior of chibaite and compare the evolution of vibrational state of guest molecules and the framework modes. On this base, we suggest the character of possible structure transformations and compare them with the available HP data on melanophlogite and synthetic MTN clathrasil.

Section snippets

Experimental

We used a sample of Arakawa chibaite from mineral collection of the National Museum of Nature and Science, Japan (NSM-M43763) with ideal formula 136SiO2·24(CH4, C2H6, C3H8, i-C4H10) [6]. Powder X-ray diffraction (XRD) patterns of a single grain of chibaite (Fig. 2) were measured prior and after the HP Raman experiment by Gandolfi camera using CuKα source (diffractometer Stoe IPDS 2T). Rietveld analysis of the XRD data for the initial sample (graph 1 in Fig. 2) was performed by RIETAN-FP [19]

Results

The XRD data of the initial and treated chibaite (graphs 2 and 3 in Fig. 2) show the preservation of crystalline structure after the HP experiment.

At ambient conditions the Raman spectrum of chibaite (Fig. 3) contains three broad bands of the framework vibrations between 100 and 400 cm−1, the bands of stretching C–C modes of hydrocarbon molecules in the region of 800–1000 cm−1, and the high-frequency stretching C–H modes along with overtones of mixed bending and stretching modes within

Discussion

The observed pressure-induced changes in the framework spectrum of chibaite are more consistent with a regular structure deformation rather than pressure-induced amorphization; in the latter case we would expect the broadening of the existing peaks, but not the appearance of new bands [27]. In addition, the splintered components of the framework bands also demonstrate regular pressure shifts. Therefore, the band splitting and bending of the ν/P curves, observed at 1.2 and 3.5 GPa, can be

Conclusions

The high-pressure Raman data show two reversible structural transformations in chibaite at about 1.2 and 3.5 GPa, associated with the distortion of framework cages and corresponding change in ordering of the guest hydrocarbon molecules. These transformations appear to not influence much a rather regular compression of structure cages throughout the whole pressure range. The chibaite structure is preserved up to 7.7 GPa without appreciable signs of amorphization, which is supported by XRD data

Acknowledgments

This work is supported by the Ministry of Education and Science of the Russian Federation (project No 14.B25.31.0032) and Russian Foundation for Basic Research (# 14-05-00616, 15-55-45070).

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