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

• The phase diagram for Fe0.9Ni0.1–H system was determined up to 15.1 GPa and 1673 K by in-situ XRD.
• The solubility of hydrogen in Fe0.9Ni0.1 was slightly lower than that in Fe.
• The melting temperature of Fe0.9Ni0.1Hx was about 100–150 K higher than that of FeHx.

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 Fe0.9Ni0.1–H system up to 15.1 GPa and 1673 K, and investigated the effect of Ni on phase relations of FeHx 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 < 0.1) as stable phases. The partial melting was observed below 6 GPa. We could not observe a double-hexagonal-close-packed (dhcp) phase because of limitations in pressure and temperature conditions. The stability field of each phase of Fe0.9Ni0.1Hx was almost same as that of FeHx. The solidus of Fe0.9Ni0.1Hx 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 Fe0.9Ni0.1Hx were depressed at around 3.5 GPa, as was the solidus of FeHx. The hydrogen contents in fcc-Fe0.9Ni0.1Hx just below solidus were slightly lower than those of fcc-FeHx, which suggests that nickel is likely to prevent dissolution of hydrogen into iron. Due to the lower hydrogen solubilities in Fe0.9Ni0.1 compared to Fe, the solidus of Fe0.9Ni0.1Hx is about 100–150 K higher than that of FeHx.