14:45 〜 15:00
[SIT18-05] First-principles calculations and neutron diffraction measurements on hydrogenation of FeS at high-pressure conditions
キーワード：FeS、水素化、中性子回折、第一原理計算
The cores of terrestrial planets consist mainly of iron and small amounts of light elements. Among the candidates for light elements, hydrogen is one of the most promising elements because of its significant effect on the volume expansion of iron by entering the interstitial sites to form iron hydride, FeHx.
The hydrogen-induced volume expansion coefficient ΔVH, representing the volume expansion per hydrogen atom in the unit cell, is an important parameter to estimate the amount of hydrogen present in the cores. To date, the hydrogen-induced volume expansion coefficients for hcp and fcc phases of iron have been determined by in-situ neutron diffraction experiments at high-temperature and high-pressure conditions.
However, there have been not many studies of hydrogenation of iron in ternary systems. Because the cores of terrestrial planets can also contain light elements other than hydrogen, it is necessary to elucidate how the presence of other light elements affects the hydrogen-induced volume expansion coefficient.
As an additional element in the system, we focus on sulfur, which is commonly found in primitive meteorites as troilite (FeS). In this study, the hydrogen-induced volume expansion coefficients of FeS, a typical iron sulfide, have been determined by static (T = 0 K) first-principles calculations using the Generalized Gradient Approximation (GGA) method. In the calculations, the hydrogen occupancies in the octahedral sites of phase V of FeS (NiAs-type phase, hereafter FeS-V) were fixed to 1 (FeSH2 in stoichiometry). We fully optimized all structural degree of freedom, the lattice parameters and the atomic coordinates of FeSH2 including stable hydrogen positions, up to 150 GPa.
The obtained ΔVH of FeS-V was notably larger than that of pure hcp iron and showed a clear negative correlation with pressure. In addition, the positions of the H atom notably deviated from the center of the octahedral sites. Because FeS is known to have a strongly correlated electron system at pressures up to ~70 GPa at 0 K (e.g., Terranova and Leeuw, 2017), where the iron magnetization survives, the GGA method may not be the best approach to estimate the positions and the volume expansion effect by the H atoms incorporation into FeS. Furthermore, the hydrogen incorporation might extend the stability field of iron magnetizations to higher pressures. In this case, using the GGA combined with the Hubbard U correction (GGA+U) is expected to improve the electronic and the atomic structures of hydrogenated magnetic FeS.
We plan to perform neutron diffraction measurements on hydrogenation of FeS above 10 GPa and determine experimentally the hydrogen-induced volume expansion coefficient of FeS. The results of the experiments and a comparison between the calculations and the experiments will be discussed in the presentation.
The hydrogen-induced volume expansion coefficient ΔVH, representing the volume expansion per hydrogen atom in the unit cell, is an important parameter to estimate the amount of hydrogen present in the cores. To date, the hydrogen-induced volume expansion coefficients for hcp and fcc phases of iron have been determined by in-situ neutron diffraction experiments at high-temperature and high-pressure conditions.
However, there have been not many studies of hydrogenation of iron in ternary systems. Because the cores of terrestrial planets can also contain light elements other than hydrogen, it is necessary to elucidate how the presence of other light elements affects the hydrogen-induced volume expansion coefficient.
As an additional element in the system, we focus on sulfur, which is commonly found in primitive meteorites as troilite (FeS). In this study, the hydrogen-induced volume expansion coefficients of FeS, a typical iron sulfide, have been determined by static (T = 0 K) first-principles calculations using the Generalized Gradient Approximation (GGA) method. In the calculations, the hydrogen occupancies in the octahedral sites of phase V of FeS (NiAs-type phase, hereafter FeS-V) were fixed to 1 (FeSH2 in stoichiometry). We fully optimized all structural degree of freedom, the lattice parameters and the atomic coordinates of FeSH2 including stable hydrogen positions, up to 150 GPa.
The obtained ΔVH of FeS-V was notably larger than that of pure hcp iron and showed a clear negative correlation with pressure. In addition, the positions of the H atom notably deviated from the center of the octahedral sites. Because FeS is known to have a strongly correlated electron system at pressures up to ~70 GPa at 0 K (e.g., Terranova and Leeuw, 2017), where the iron magnetization survives, the GGA method may not be the best approach to estimate the positions and the volume expansion effect by the H atoms incorporation into FeS. Furthermore, the hydrogen incorporation might extend the stability field of iron magnetizations to higher pressures. In this case, using the GGA combined with the Hubbard U correction (GGA+U) is expected to improve the electronic and the atomic structures of hydrogenated magnetic FeS.
We plan to perform neutron diffraction measurements on hydrogenation of FeS above 10 GPa and determine experimentally the hydrogen-induced volume expansion coefficient of FeS. The results of the experiments and a comparison between the calculations and the experiments will be discussed in the presentation.