4:40 PM - 4:55 PM
[SIT18-10] Investigating the Superionic Inner Core Through In-situ High-temperature Compression Experiments On Iron Hydride
Keywords:High-pressure experiment, Iron hydride, Superionic inner core
The Earth's inner core, the center of the planet, plays a crucial role in understanding the Earth's internal dynamics and thermal history. Despite its importance, the crystal structure, chemical composition, and thermal state of the inner core still remain largely uncertain. Several anomalous seismic observations, such as the low shear velocity and the complex anisotropic seismic structure, have not been fully explained by previous studies with various Fe(–Ni)–light element alloys. Recent ab initio calculations argue that the inner core is in a superionic state, where light elements in the Fe–Ni alloy can diffuse rapidly (>10−10 m2/s). Alloys in the superionic state exhibit peculiar physical properties, such as elastic weakening, that are harmonious with seismic observations of the inner core. However, the superionic state in Fe-based alloys at high pressures and temperatures has not yet been verified by experiments.
To investigate the solid-superionic transition in face-centered cubic (fcc) FeHx (x ~ 1), we collected dense data set of synchrotron diffraction measurements in a pressure range between 50 and 85 GPa at high temperatures (300–2200 K) generated in a laser-heated diamond anvil cell. We observed a temporary increase in lattice volume within the temperature range of 1500–1800 K, followed by drastic lattice distortion at higher temperatures. These phenomena share similarities with those observed in studies of the solid-superionic transition of fluorite-type materials, such as PbF2, which are structurally similar to fcc FeHx.
The solid-superionic phase boundary at the current pressure range is defined by a linear function of pressure, yielding T=6.6P+1260, where T and P are temperature and pressure in K and GPa, respectively. While, the presented phase boundary exhibits a greater temperature dependence compared to previous reports based on ab initio calculations, its linear extrapolation predicts that fcc FeHx (x ~ 1) is in a superionic state corresponding to the Earth's interior, from the lower mantle to the inner core. This prediction is consistent with the previous ab initio calculations. The potential causes of these observed phenomena corresponding to the solid-superionic transition and their geophysical implications are discussed in reference to fluorite-type superionic materials. Further investigation into the superionicity of Fe–Ni–light element alloys is potentially crucial to resolve the observed anomalies in the Earth's inner core.
To investigate the solid-superionic transition in face-centered cubic (fcc) FeHx (x ~ 1), we collected dense data set of synchrotron diffraction measurements in a pressure range between 50 and 85 GPa at high temperatures (300–2200 K) generated in a laser-heated diamond anvil cell. We observed a temporary increase in lattice volume within the temperature range of 1500–1800 K, followed by drastic lattice distortion at higher temperatures. These phenomena share similarities with those observed in studies of the solid-superionic transition of fluorite-type materials, such as PbF2, which are structurally similar to fcc FeHx.
The solid-superionic phase boundary at the current pressure range is defined by a linear function of pressure, yielding T=6.6P+1260, where T and P are temperature and pressure in K and GPa, respectively. While, the presented phase boundary exhibits a greater temperature dependence compared to previous reports based on ab initio calculations, its linear extrapolation predicts that fcc FeHx (x ~ 1) is in a superionic state corresponding to the Earth's interior, from the lower mantle to the inner core. This prediction is consistent with the previous ab initio calculations. The potential causes of these observed phenomena corresponding to the solid-superionic transition and their geophysical implications are discussed in reference to fluorite-type superionic materials. Further investigation into the superionicity of Fe–Ni–light element alloys is potentially crucial to resolve the observed anomalies in the Earth's inner core.