17:15 〜 19:15
[SIT18-P03] 六方最密充填構造重水素化鉄の水素位置と水素誘起体積膨張-地球内核の水素含有量の推定-
キーワード：水素、鉄、高温高圧
Hydrogen is considered one of the candidate light elements in the Earth's core. Under high pressure, iron reacts with hydrogen to form iron hydrides, where hydrogen atoms occupy interstitial sites in the metal lattice. Iron hydrides have been studied for a long time, and the PT phase diagram of the Fe-H system has been established up to 20 GPa and 1500 K. The stable phases take various metal lattices of bcc, fcc, hcp and double hcp (dhcp). The hcp-iron hydride is stable only for compositions of FeHx (x < ~0.5) and is believed to exist over a wide temperature range, including Earth's inner core conditions.
Understanding the site occupancy of hydrogen atoms is crucial for determining the characteristics of iron hydrides. The hcp metal lattice has octahedral (O) and tetrahedral (T) interstitial site, which can accommodate hydrogen atoms. Previous studies [1, 2] have reported that hydrogens exclusively occupy O-sites. However, this conclusion is based on the results of Rietveld analysis of neutron diffraction profiles in which hcp iron coexists with other phases. Furthermore, the pressure and temperature range of the experiments were limited to 5.1 GPa and 673 K, respectively. The hydrogen-induced volume expansion calculated in the previous studies, ranges from 2.2 to 2.7 Å3/D-atom.
In this study, we expanded the pressure range of the investigation by developing high pressure cells and conducted neutron diffraction experiments under high-PT conditions to clarify the deuterium position in hcp lattice and the PT dependence of hydrogen-induced volume expansion.
Neutron diffraction experiments under high PT conditions were performed at BL11 PLANET beamline in J-PARC MLF. The six-axis multi-anvil press, ATSUHIME, was used as to generate high PT conditions. The sample was compressed with the MA6-8 method using WC anvils. The sample was deuterated with deuterated ammonia borane (ND3BD3) under high PT conditions. The hcp-FeDx was synthesized by cooling the sample to room temperature after confirming deuterium release. We conducted two experiments with different initial D/Fe ratios of 0.46 and 0.60. The long-exposure were performed at PT conditions of 10 to 16 GPa and 300 to 1100 K during heating process. The site occupancies and atomic position of hydrogen were determined by the Rietveld refinement of the obtained neutron diffraction profiles.
The site occupancy of deuterium was analyzed using two models with different deuterium occupations of i) only O site occupation (O-site model) and ii) both O- and T-site occupation (OT-site model). The refinement results for both models showed almost the same reliability factors of Rwp and S. The T-site occupancy in OT-site model is less than 0.01, indicating that the O-site model can adequately describe the deuterium occupancy. The hydrogen-induced volume expansion increases with temperature. Taking the temperature dependence into account, the values in the previous study were consistent with this study. The hydrogen content in the inner core is estimated by extrapolating PT dependence of hydrogen-induced volume expansion to Earth's inner core conditions (330 GPa, 5500 K). Assuming that the hydrogen is the only light element in the inner core, the hydrogen content of the Earth’s inner core is estimated to be approximately 0.1 wt%.
References
1] V.E. Antonov, K. Cornell, V.K. Fedotov, A.I. Kolesnikov, E.G. Ponyatovsky, V.I. Shiryaev, H. Wipf: J. Alloy. Compd. 264, 214 (1998)
[2] A. Machida, H. Saitoh, T. Hattori, A. Sano-Furukawa, K. Funakoshi, T. Sato, S. Orimo, K. Aoki: Sci. Rep, 9, 12290 (2019).
Understanding the site occupancy of hydrogen atoms is crucial for determining the characteristics of iron hydrides. The hcp metal lattice has octahedral (O) and tetrahedral (T) interstitial site, which can accommodate hydrogen atoms. Previous studies [1, 2] have reported that hydrogens exclusively occupy O-sites. However, this conclusion is based on the results of Rietveld analysis of neutron diffraction profiles in which hcp iron coexists with other phases. Furthermore, the pressure and temperature range of the experiments were limited to 5.1 GPa and 673 K, respectively. The hydrogen-induced volume expansion calculated in the previous studies, ranges from 2.2 to 2.7 Å3/D-atom.
In this study, we expanded the pressure range of the investigation by developing high pressure cells and conducted neutron diffraction experiments under high-PT conditions to clarify the deuterium position in hcp lattice and the PT dependence of hydrogen-induced volume expansion.
Neutron diffraction experiments under high PT conditions were performed at BL11 PLANET beamline in J-PARC MLF. The six-axis multi-anvil press, ATSUHIME, was used as to generate high PT conditions. The sample was compressed with the MA6-8 method using WC anvils. The sample was deuterated with deuterated ammonia borane (ND3BD3) under high PT conditions. The hcp-FeDx was synthesized by cooling the sample to room temperature after confirming deuterium release. We conducted two experiments with different initial D/Fe ratios of 0.46 and 0.60. The long-exposure were performed at PT conditions of 10 to 16 GPa and 300 to 1100 K during heating process. The site occupancies and atomic position of hydrogen were determined by the Rietveld refinement of the obtained neutron diffraction profiles.
The site occupancy of deuterium was analyzed using two models with different deuterium occupations of i) only O site occupation (O-site model) and ii) both O- and T-site occupation (OT-site model). The refinement results for both models showed almost the same reliability factors of Rwp and S. The T-site occupancy in OT-site model is less than 0.01, indicating that the O-site model can adequately describe the deuterium occupancy. The hydrogen-induced volume expansion increases with temperature. Taking the temperature dependence into account, the values in the previous study were consistent with this study. The hydrogen content in the inner core is estimated by extrapolating PT dependence of hydrogen-induced volume expansion to Earth's inner core conditions (330 GPa, 5500 K). Assuming that the hydrogen is the only light element in the inner core, the hydrogen content of the Earth’s inner core is estimated to be approximately 0.1 wt%.
References
1] V.E. Antonov, K. Cornell, V.K. Fedotov, A.I. Kolesnikov, E.G. Ponyatovsky, V.I. Shiryaev, H. Wipf: J. Alloy. Compd. 264, 214 (1998)
[2] A. Machida, H. Saitoh, T. Hattori, A. Sano-Furukawa, K. Funakoshi, T. Sato, S. Orimo, K. Aoki: Sci. Rep, 9, 12290 (2019).