Photon Factory Activity Report 2017 #35 (2018)

BL-18C/2016G523

In-situ X-ray diffraction studies of the stability of methane hydrate
under high pressure and high temperature
Hirokazu Kadobayashi*1, Hiroaki Ohfuji1, Hisako Hirai2, Michika Ohtake3, Yoshitaka Yamamoto3
1. Geodynamics Research Center, Ehime University, 2. Department of Environment Systems, Facultyof Geo-environmental Science, Rissho University, 3. National Institute of Advanced Industrial
Science and Technology
1 Introduction
Methane hydrate is thought to be an important
constituent of icy planets and their satellites, such as
Neptune, Uranus and Titan. It is a clathrate compoundcomposed of hydrogen-bonded water cages andmethane molecules included in the cages. Methane
hydrate has an sI cage structure composed of two 12hedral and six 14-hedral components in a unit cell at
low pressures (< 0.8 GPa) and room temperature. It
transforms to an sH cage structure composed of three
12-hedral, two modified 12-hedral, and a 20-hedral atapproximately 0.8 GPa, which further transforms to afilled-ice Ih structure at approximately 1.8 GPa1-3.
Although the sequence of the phase transitions withpressure have been studied well at room temperature,
there are only a few studies that addressed the stabilityof methane hydrate under high pressure and hightemperature4, 5. In addition, the pressure range of theseprevious studies is only limited to < 5 GPa. Therefore,
a further investigation is needed to understand thestability and physicochemical behavior of methane
hydrate under extreme conditions corresponding to themantle of giant icy planets.
2 Experiment
Lever-spring-type diamond anvil cell was used
for high pressure generation. Pressure was estimatedby using the ruby fluorescence method. Hightemperature generation was achieved by an electricfurnace. Temperature was measured with an accuracyof ±1 K by a chromel-alumel thermocouple. The
starting material, methane hydrate sI sample, wassynthesized by the conventional ice–gas interface
reaction method at a fixed P-T condition of 15 MPaand 271 K at the National Institute of Advanced
Industrial Science and Technology. The powderedsample of methane hydrate was loaded together with afew ruby balls as a pressure marker in a sample hole ina Re gasket in a cryogenic vessel cooled by liquidnitrogen. In-situ XRD observations were performed at
BL-18C beamline of the Photon Factory of the HighEnergy Accelerator Research Organization (KEK),
where monochromatic X-rays with wavelength of0.06198 nm was available.

3 Results and Discussion
Fig. 1 shows the typical XRD patterns before
and after heating at around 20 GPa. All diffractionlines observed were indexed with the filled ice Ih(orange circles), solid methane (purple cross) and ice
VII (blue triangle). Before heating, the typical XRD
patterns of filled ice Ih and ice VII were observed at298 K and 18.3 GPa. After heating, the XRD patternsof filled ice Ih survived at least 423 K and 20.0 GPa,but the diffraction patterns of filled ice Ih disappeared
and were replaced by those of solid methane at 503 K
and 15.5 GPa. This means that filled ice Ihdecomposes into solid methane and ice VII above 423
K at around 20.0 GPa. The results of high P-Texperiments showed that filled ice Ih decomposes into
solid methane and ice VII at temperatures considerably
lower than the melting curves of solid methane and iceVII at around 20 GPa. This suggests that methane
hydrate is unlikely to be stable in the mantle of gianticy planets such as Neptune and Uranus.

Fig. 1: Representative XRD patterns before and after
heating at around 20 GPa. Orange circles, purple crossand blue triangle indicate the diffraction lines of filled
ice Ih, solid methane and Ice VII, respectively.

Photon Factory Activity Report 2017 #35 (2018)

Acknowledgement
This research was partly supported by a Grant-inAid for JSPS Fellows (Grant No. 17J05467) from theJapan Society for the Promotion of Science.
References[1] H. Hirai et al., J. Chem. Phys. 115,7066 (2001)[2] J. S. Loveday et al., Nature. 410,661 (2001)[3] H. Shimizu et al., J. Phys. Chem. B. 106,30 (2002)[4] A. Kurnosov et al. Z. Naturforsch. 61b, 1573
(2006)[5] L. Bezacier et al. Phys. Earth Planetary interiors
229, 144 (2014)*kadobayashi@sci.ehime-u.ac.jp