Unconventional iron-magnesium compounds at terapascal pressures

Being a lithophile element at ambient pressure, magnesium has been long believed to be immiscible with iron. Authors of a recent study showed that pressure turns magnesium into a siderophile element and can produce unconventional Fe-Mg compounds [Gao et al., New J. Chem. 43, 17403 (2019)]. Here, we extend the investigation to exoplanetary pressure conditions using an adaptive genetic algorithm-based variable-composition structural prediction approach. We identify several Fe-Mg phases up to 3 TPa at 0 K. Our cluster alignment analysis reveals that most of the predicted Fe-Mg compounds prefer a body-centered cubic packing motif at terapascal pressures. In this paper, we provide a more comprehensive structure database to support future investigations of the high-pressure structural behavior of Fe-Mg and ternary, quaternary, etc. compounds involving these elements.

Figure 1

Stability of Fe-Mg compounds. (a) Convex hull diagrams of the Fe-Mg compounds at exoplanetary pressures. Solid symbols represent the ground states, while open symbols denote the metastable phases. The convex hulls are shown by solid lines that connect ground states. (b) Pressure-composition phase diagram of the Fe-Mg system.

Figure 2

(a) Crystal structure of $I4/mmm {\mathrm{Fe}}_2\mathrm{Mg}$ at 1 TPa. Fe and Mg atoms are indicated by brown and green balls, respectively. (b) Fe-centered and (c) Mg-centered coordination polyhedra. (d) Phonon dispersion of $I4/mmm {\mathrm{Fe}}_2\mathrm{Mg}$ at 1 TPa.

Figure 3

(a) Crystal structure of $Pm\overline{3}m$ FeMg. Fe and Mg atoms are indicated by brown and green balls, respectively. (b) Mg-centered coordination polyhedra. (c)–(e) Phonon spectra of $Pm\overline{3}m$ FeMg at 1, 2, and 3 TPa, respectively.

Figure 4

(a) Crystal structure of $P{6}_3/mmc \mathrm{Fe}{\mathrm{Mg}}_2$ at 1 and 2 TPa. Fe and Mg atoms are indicated by brown and green balls, respectively. The dashed line indicates the lattice analogous to the well-known ω phase. (b) Fe-centered and (c) Mg-centered coordination polyhedra. (d) and (e) Phonon dispersions of $P{6}_3/mmc \mathrm{Fe}{\mathrm{Mg}}_2$ at 1 and 2 TPa, respectively. (f) Crystal structure of $P6/mmm \mathrm{Fe}{\mathrm{Mg}}_2$ at 3 TPa. (g) Fe-centered and (h) Mg-centered coordination polyhedra, red dashed lines indicate a prismatic wedge. (i) Phonon dispersion of $P6/mmm \mathrm{Fe}{\mathrm{Mg}}_2$ at 3 TPa.

Figure 5

(a) Crystal structure of $Fm\overline{3}m \mathrm{Fe}{\mathrm{Mg}}_3$ at 1 TPa. Fe and Mg atoms are indicated by brown and green balls, respectively. (b) Fe-centered and (c) Mg-centered coordination polyhedra. (d) Phonon dispersion of $Fm\overline{3}m \mathrm{Fe}{\mathrm{Mg}}_3$ at 1 TPa.

Figure 6

The relative enthalpies ($H_d$) of low-enthalpy ${\mathrm{Fe}}_x{\mathrm{Mg}}_y$ structures as a function of their volumes, where the symbols represent the local packing motifs, the colors denote the atomic content of Mg. The local bonding states of the template motifs are shown on the right. The label “others” indicates a Fe-centered cluster with the lowest alignment scores of all five templates >0.125.

Figure 7

Relative volume as a function of pressure for elementary Fe and Mg phases. The inset shows the same at low pressures.