Helium-Iron Compounds at Terapascal Pressures

We investigate the binary phase diagram of helium and iron using first-principles calculations. We find that helium, which is a noble gas and inert at ambient conditions, forms stable crystalline compounds with iron at terapascal pressures. A FeHe compound becomes stable above 4 TPa, and a ${\mathrm{FeHe}}_2$ compound above 12 TPa. Melting is investigated using molecular dynamics simulations, and a superionic phase with sublattice melting of the helium atoms is predicted. We discuss the implications of our predicted helium-iron phase diagram for interiors of giant (exo)planets and white dwarf stars.

Figure 1

Pressure-composition phase diagram of the helium-iron system at the static lattice level. For FeHe, the $Cmcm$ structure is stable between 4 and 50 TPa, and the $Fm\overline{3}m$ structure above 50 TPa. For ${\mathrm{FeHe}}_2$, the $Cmmm$ structure is stable between 12 and 47 TPa, and above that pressure the stable structure is $I{4}_1/amd$. The formation energy per atom, calculated using $\mathrm{\Delta }{\mathcal{G}}_s=[{\mathcal{G}}_s-({\mathcal{G}}_{\mathrm{He}}{N}_{\mathrm{He}}+{\mathcal{G}}_{\mathrm{Fe}}{N}_{\mathrm{Fe}})]/(N_{\mathrm{He}}+N_{\mathrm{Fe}})$ as described in the text, is indicated by the gradients and approaches $-4\mathrm{eV}/\mathrm{atom}$ for both stoichiometries at 100 TPa.

Figure 2

Crystal structures of $Cmcm$ FeHe and $Cmmm$ ${\mathrm{FeHe}}_2$ at 10 TPa. Helium atoms are represented in blue, and iron atoms in gray.

Figure 3

Helium-iron phase diagram. The solid lines indicate formation lines, sublattice melting, and full melting. The sublattice melting of FeHe occurs at $15100\pm 1000\mathrm{K}$ at 5.38 TPa, and at $18200\pm 1000\mathrm{K}$ at 10.6 TPa, while the full melting of FeHe occurs at $17600\pm 1000\mathrm{K}$ at 5.45 TPa, and at $19800\pm 1000\mathrm{K}$ at 10.65 TPa. The melting line of iron is taken from Ref. [39].