9:20 AM - 9:35 AM
[SIT18-03] Interstitial site occupancy of deuterium in a Fe-Si binary system at high-pressure and high-temperature conditions using a large volume press
Keywords:high-pressure and high-temperature experiments, multi anvil press, Hydrogenation, Iron alloy, light element, neutron diffraction
Light elements in the Earth's core have been discussed for a long time as one of the most important issues in earth science. Hydrogen, the most abundant element in the solar system, is one of the most promising candidates for the light elements because it can dissolve into metals under high-pressure conditions (e.g., Fukai et al.,1984) and can significantly lower the melting point of iron (e.g., Sakamaki et al., 2009). The second candidate is silicon. A certain amount of Si has been considered to be dissolved into the core from geochemical studies such as the difference in 30Si/28Si ratio (e.g., Georg et al., 2007) between meteorites and terrestrial rocks, higher Mg/Si ratio of Earth’s mantle than the solar abundance, and so on (Allegre et al., 1995).
Hydrogen contained in iron is unquenchable to ambient condition and it is impossible to observe hydrogen in iron by X-ray diffraction. In order to make up those shortcomings, neutron diffraction experiments on iron deuteride under high pressure and high temperature have been conducted (Machida et al., 2014, Machida et al., 2019, Saitoh et al., 2020) . Those studies have determined site occupancies of deuterium in FeDX with fcc, hcp and dhcp structures. The changes in H dissolution and site occupancy when Fe contains other light elements will be a next frontier to be elucidated.
In this study, we focused on Si and H as the candidates of light elements. Two kind of starting materials, Fe0.95Si0.05 and FeSi were used for the experiments. At ambient condition, Fe0.95Si0.05 takes bcc structure and FeSi takes B20 structure. X-ray diffraction experiments under high-pressure and high-temperature conditions (6 – 15 GPa, 300 – 1000 K) were carried out at beamline NE7, PF-AR, KEK to estimate the pressure and temperature dependence on hydrogen amount in the Fe-Si binary systems. Amount of dissolved hydrogen in the unit cell was estimated by dividing the volume expansion by the volume expansion rate. In addition to X-ray diffraction experiments, neutron diffraction experiments were conducted on Fe-Si-D ternary system at high pressure and high temperature. Those experiments were carried out using the 6 axis multi-anvil press "ATSUHIME" installed at BL11, MLF, J-PARC. ND3BD3 was used for deuterium source. Interstitial site occupancy of deuterium in the ternary alloy was determined from Rietveld refinement on neutron diffraction data of fcc Fe0.95Si0.05Dx (starting x ~ 2) At 7.2 GPa and 988 K, x value of Fe0.95Si0.05Dx was 0.73 and deuterium atoms occupy not only the octahedral site (gD(O) = 0.700) but also the tetrahedral site (gD(T) = 0.014). Machida et al., (2014) reported that gD(O) = 0.700 and gD(T) = 0.056 in fcc FeDx under the close P-T condition. In this study, it was clarified that the site occupancy in the tetrahedral site of Fe0.95Si0.05 alloy is much smaller than that of FeDx. Compared with previously reported neutron diffraction experiments on FeDx under high pressure and high temperature, the coexistence of silicon can affect the site occupancies of hydrogen. In addition, we conducted a long-time measurement at ~ 14.5 GPa and 900 K using the MA6-8 assembly for neutron diffraction. The diffraction pattern shows that bcc iron completely transformed to hcp phase, which is the same phase of iron we believe to be present in the inner core. The deuterium content x in hcp-Fe0.95Si0.05Dx was clarified to be 0.40 at ~ 14.7 GPa and 800 K.
Hydrogen contained in iron is unquenchable to ambient condition and it is impossible to observe hydrogen in iron by X-ray diffraction. In order to make up those shortcomings, neutron diffraction experiments on iron deuteride under high pressure and high temperature have been conducted (Machida et al., 2014, Machida et al., 2019, Saitoh et al., 2020) . Those studies have determined site occupancies of deuterium in FeDX with fcc, hcp and dhcp structures. The changes in H dissolution and site occupancy when Fe contains other light elements will be a next frontier to be elucidated.
In this study, we focused on Si and H as the candidates of light elements. Two kind of starting materials, Fe0.95Si0.05 and FeSi were used for the experiments. At ambient condition, Fe0.95Si0.05 takes bcc structure and FeSi takes B20 structure. X-ray diffraction experiments under high-pressure and high-temperature conditions (6 – 15 GPa, 300 – 1000 K) were carried out at beamline NE7, PF-AR, KEK to estimate the pressure and temperature dependence on hydrogen amount in the Fe-Si binary systems. Amount of dissolved hydrogen in the unit cell was estimated by dividing the volume expansion by the volume expansion rate. In addition to X-ray diffraction experiments, neutron diffraction experiments were conducted on Fe-Si-D ternary system at high pressure and high temperature. Those experiments were carried out using the 6 axis multi-anvil press "ATSUHIME" installed at BL11, MLF, J-PARC. ND3BD3 was used for deuterium source. Interstitial site occupancy of deuterium in the ternary alloy was determined from Rietveld refinement on neutron diffraction data of fcc Fe0.95Si0.05Dx (starting x ~ 2) At 7.2 GPa and 988 K, x value of Fe0.95Si0.05Dx was 0.73 and deuterium atoms occupy not only the octahedral site (gD(O) = 0.700) but also the tetrahedral site (gD(T) = 0.014). Machida et al., (2014) reported that gD(O) = 0.700 and gD(T) = 0.056 in fcc FeDx under the close P-T condition. In this study, it was clarified that the site occupancy in the tetrahedral site of Fe0.95Si0.05 alloy is much smaller than that of FeDx. Compared with previously reported neutron diffraction experiments on FeDx under high pressure and high temperature, the coexistence of silicon can affect the site occupancies of hydrogen. In addition, we conducted a long-time measurement at ~ 14.5 GPa and 900 K using the MA6-8 assembly for neutron diffraction. The diffraction pattern shows that bcc iron completely transformed to hcp phase, which is the same phase of iron we believe to be present in the inner core. The deuterium content x in hcp-Fe0.95Si0.05Dx was clarified to be 0.40 at ~ 14.7 GPa and 800 K.