High-pressure and high-temperature phase diagram for Fe0.9Ni0.1–H alloy

Abstract Planetary cores are considered to consist of an iron–nickel (Fe–Ni) alloy and light elements and hydrogen is one of plausible light elements in the core. Here we have performed in situ X-ray diffraction experiments on an Fe 0.9 Ni 0.1 –H system up to 15.1 GPa and 1673 K, and investigated the effect of Ni on phase relations of FeH x under high pressure and high temperature. The experimental system in the present work was oversaturated with hydrogen. We found a face-center-cubic ( fcc ) phase (with hydrogen concentration up to x ∼1) and a body-center-cubic ( bcc ) phase ( x  dhcp ) phase because of limitations in pressure and temperature conditions. The stability field of each phase of Fe 0.9 Ni 0.1 H x was almost same as that of FeH x . The solidus of Fe 0.9 Ni 0.1 H x was 500–700 K lower than the melting curve of Fe and its liquidus was 400–600 K lower than that of Fe in the pressure range of this study. Both the solidus and liquidus of Fe 0.9 Ni 0.1 H x were depressed at around 3.5 GPa, as was the solidus of FeH x . The hydrogen contents in fcc -Fe 0.9 Ni 0.1 H x just below solidus were slightly lower than those of fcc -FeH x , which suggests that nickel is likely to prevent dissolution of hydrogen into iron. Due to the lower hydrogen solubilities in Fe 0.9 Ni 0.1 compared to Fe, the solidus of Fe 0.9 Ni 0.1 H x is about 100–150 K higher than that of FeH x .