Fig. 1. (a)–(c) Ternary phase diagrams of the Fe–F–He, Fe–F–Xe, and Fe–Cl–Xe systems at selected pressures and 0 K. Five thermodynamically stable ternary compounds have been identified. (d) Composition–pressure phase diagram of the five ternary compounds over a wide range of pressure and at 0 K.

Fig. 2. Crystal structures of stable ternary compounds: (a) Fm-3m-FeF₂He; (b) P6₃/m-FeF₃He; (c) P2₁/m-FeF₂Xe; (d) Pmmn-FeF₃Xe; (e) P6₃/mmc-FeCl₃Xe. The lattice parameters of these compounds are given in Table S3 (supplementary material).

Fig. 3. Pressure–temperature (P–T) phase diagrams of the Fe–F–He and Fe–X–Xe (X = F, Cl) compounds FeF₂He (a), FeF₃He (b), FeF₂Xe (c), FeF₃Xe (d), and FeCl₃Xe (e). The lines connecting black squares indicate phase boundaries. Green circles, blue triangles, and red diamonds represent the solid, sublattice melting and liquid phases, respectively, dividing P–T space into the same-color shaded regions for these distinct states of matter. The geotherm of the present Earth is indicated by the thick orange line.

Fig. 4. (a)–(c) Mean-square displacements (MSDs) of atoms in P6₃/m-FeF₃He from AIMD simulations: (a) there is no atomic diffusion at 1500 K and 52 GPa in the solid phase; (b) He atoms are diffusive at 2000 K and 57 GPa in the sublattice melting phase; (c) all atoms become diffusive at 3000 K and 65 GPa in the liquid phase. (d)–(f) Trajectories showing the corresponding real-space atomic displacements over the simulation time span.