Effect of nickel on the high-pressure phases in FeH

Hydrogen-rich metal alloys and compounds have drawn interest from planetary geophysics and condensed matter physics communities because of their potential for deep hydrogen storage in planets and high-temperature superconductivity. We find that a small amount of Ni can alter the phase behavior in the FeH alloy system. Ni can stabilize the double hexagonal close-packed (dhcp) structure in FeH up to the liquidus at 33 GPa, which is in contrast with the stability of the face-centered cubic (fcc) structure in Ni-free FeH at the same conditions. Above 60 GPa, Ni suppresses the stability of the tetragonal ${\mathrm{FeH}}_{2}$ phase but stabilizes fcc FeH at higher temperatures. At the same pressure range, we find tetragonal ${\mathrm{FeH}}_{2}$ and cubic ${\mathrm{FeH}}_{3}$ to be stable at temperatures above 2500 K without Ni. Therefore, in planetary interiors, Ni will expand the stability field of dense close-packed structures in the FeH system. If the Ni content is low, then ${\mathrm{FeH}}_{2}$ and ${\mathrm{FeH}}_{3}$ can play an important role in the cores of hydrogen-rich planets. Also, our study demonstrates that a secondary alloying component can severely impact the high-pressure stability of polyhydrides.