Photon Factory Activity Report 2019 #37 (2020)

BL-18C/2018G524

Retention of filled-ice structure of methane hydrate up to 134 GPa
Hirokazu KADOBAYASHI1,* Hisako HIRAI2, and Yoshitaka YAMAMOTO3
1 National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan

2 Faculty of Geo-environmental Science, Rissho University, Kumagaya, Saitama 360-0194, Japan
3 National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8569,

Japan

1 Introduction
Methane hydrate (MH) is one of the well-studied
clathrate hydrates, which consist of host cages
formed by hydrogen-bonded water molecules andguest methane molecules included in the cagestructures [1]. It presents an sI cage structure (MH-sI)
below 0.8 GPa and at room temperature [1]. At 0.8
GPa, the sI cage structure of MH becomes an sH
cage structure (MH-sH), and at 1.8 GPa, a filled-ice
Ih cage structure (MH-FI) is formed [2–5]. MH-FI
presents an ice framework with a structure similar tothat of ice Ih in which guest methane molecules canbe entrapped [2]. At higher pressure, the fundamentalstructure of MH-FI is retained up to at least 86 GPa,with some phase transitions occurring at 15–20 GPa
and 40–50 GPa [6–9]. The 15–20 GPa transition,from MH-FI to MH-GOS (GOS: guest-ordered state),
was detected via Raman spectroscopy and X-ray
powder diffraction (XRD) and interpreted to becaused by the orientational ordering of guest
methane molecules within the framework [9]. The 40–50 GPa transition, from MH-GOS to a high-pressure
phase (MH-HP), was accompanied by theappearance of new XRD lines whose d-values wereclose to those of MH-GOS [7,9], suggesting that thestructure of MH-HP may be related to that of MHGOS. The transition from MH-FI to MH-GOS is wellunderstood [9]. On the other hand, that from MHGOS to MH-HP has not yet been comprehensively
investigated although its existence was detected fromthe appearance of new diffraction lines [6–9]. That ismainly because the latter transition is significantly
slow, and both MH-GOS and MH-HP coexist underwide pressure ranges. Thus, the diffraction lines
originating exclusively from MH-HP have not yet been
identified owing to the superimposition of lines fromboth phases. Therefore, to understand themechanism of MH-GOS to MH-HP transition, it isfirstly required to identify the characteristic diffractionlines of MH-HP. In this study, high-pressure
experiments (up to 134 GPa) were performed in orderto clarify the changes in the host framework during
individual transitions.
2 ExperimentHigh pressure was generated by using diamondanvil cells with culet sizes of 100 to 350 μm. Rhenium
was used as the gasket material. The sample

pressure was estimated based on the diamond
Raman shift [10] and the ruby fluorescence methods[11]. The starting material, MH-sI sample, wassynthesized via the conventional ice–gas interface
reaction method [12] at the National Institute ofAdvanced Industrial Science and Technology. The
powdered MH-sI sample (average grain size of 2–3µm) was loaded into the sample chamber in acryogenic vessel cooled by liquid nitrogen together
with a few ruby balls (ca. 2–10 µm) used as pressure
markers. XRD experiments were performed using
beamline BL-10XU at SPring-8 and beamline BL-18C
at Photon Factory, High Energy Accelerator
Research Organization (KEK), where monochromaticX-rays with wavelengths of 0.04424 and 0.06121 nm
were used, respectively. The pressure range for XRD
was 1.8–134 GPa. All experiments were performed atroom temperature (298 K).
3 Results and Discussion
In order to clearly distinguish the diffraction lines ofMH-GOS from those of MH-HP, the transition fromMH-GOS to MH-HP under increasing pressure wascarefully investigated. Figs. 1(a) and 1(b) show therepresentative XRD patterns during MH-GOS to MHHP transition and the variations of the d-values fromthe individual phases with pressure, respectively.
In Fig. 1(a), the XRD spectrum at 26.1 GPa exhibitsa typical MH-GOS pattern; all diffraction lines wereindexed as MH-GOS and ice VII (indicated with green
circles and blue rhombi, respectively). With
increasing pressure, three new diffraction lines(indicated with red circles) that belonged to neither
MH-GOS nor solid methane began to appear at 33.8
GPa. The intensities of these new diffraction lines
gradually increased, and, in contrast, those of MHGOS gradually decreased and remained visible in thespectrum up to 53.7 GPa. At 53.7 GPa, two additional
diffraction lines appeared. Finally, the diffraction linesof MH-GOS were totally replaced by five new
diffraction lines when pressure reached 57.7 GPa.
The d-values of the new diffraction lines were close
to those of MH-GOS at 33.8–53.7 GPa (Fig. 1(b)).
The d-values of the new diffraction lines weredifferent from those for solid methane reported inprevious studies [13,14]. By analyzing the transition
process using the approach proposed herein, fivetypical diffraction lines of MH-HP could be identified.

Photon Factory Activity Report 2019 #37 (2020)

These new lines were in excellent agreement withthose for a structure with space group Pmcn
predicted recently by a theoretical study [15]. The
identification of the diffraction lines exclusive to MH(a)133.8 GPa110.2 GPa82.2 GPa

Intensity (a.u.)

67.5 GPa
57.7 GPa
53.7 GPa
48.3 GPa

33.8 GPa26.1 GPa

1.2

1.7

2.2

2.7

3.2

3.7

d-value (Å)(b)

MH-GOS

3.7

d-value (Å)

110

3.2002

2.7

121

2.2

112
132

1.7

123

1.2
20

40

60

80

100120140

Pressure (GPa)FIG. 1. (a) Typical XRD patterns of MH and ice VII
coexisting in the sample chamber. Red circles,green circles, blue rhombi, and purple triangles
indicate MH-HP, MH-GOS, ice VII, and Rhenium,respectively. (b) Variation of d-values withpressure. Open green and solid red symbols
indicate the d-values of MH-GOS and MH-HP,respectively.
HP allowed us to identify the start of the transition
more accurately, which was found to occur at
approximately 34 GPa (6 GPa lower than thepreviously reported value) [6–9]. To explore thepossibility of MH-HP undergoing a further transition,
XRD experiments up to 134 GPa were performed. Nochanges such as diffraction line splitting or new
diffraction line appearance were observed above
57.7 GPa after the transition to MH-HP wascompleted; i.e., MH-HP is stable up to at least 134
GPa without presenting any assessable change (Fig.
1 (a)).
Acknowledgement
This research was partly supported by a Grant-inAid for JSPS Fellows (grant no. 19J01467) and EarlyCareer Scientists (grant no. 19K14815) from theJapan Society for the Promotion of Science. This
research was also supported by the Joint
Usage/Research Center PRIUS, Ehime University,Japan.
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* KADOBAYASHI.Hirokazu@nims.go.jp