Synthesis of Xenon and Iron-Nickel Intermetallic Compounds at Earth’s Core Thermodynamic Conditions

Using in situ synchrotron x-ray diffraction and Raman spectroscopy in concert with first principles calculations we demonstrate the synthesis of stable $\mathrm{Xe}(\mathrm{Fe},\mathrm{Fe}/\mathrm{Ni}{)}_3$ and ${\mathrm{XeNi}}_3$ compounds at thermodynamic conditions representative of Earth’s core. Surprisingly, in the case of both the Xe-Fe and Xe-Ni systems Fe and Ni become highly electronegative and can act as oxidants. The results indicate the changing chemical properties of elements under extreme conditions by documenting that electropositive at ambient pressure elements could gain electrons and form anions.

Figure 1

(a) XRD patterns of the $\mathrm{Xe}\text{−}{\mathrm{Fe}}_{0.93}$ ${\mathrm{Ni}}_{0.07}$ mixture before and after laser heating at 210 GPa. New Bragg peaks after laser heating and the corresponding Miller indices for the Pm-3m and the Pmmn ${\mathrm{XeFe}}_3$ crystal structures are noted with green and blue dots, respectively. The peak marked by the asterisk corresponds to the strongest peak of rhenium (gasket material). (b) Le Bail refinement of the $\mathrm{Xe}({\mathrm{Fe}}_{0.93}{\mathrm{Ni}}_{0.07}{)}_3$ compound at 210 GPa. The peaks of the Pm-3m and Pmmn (1) structures are marked with green and blue vertical lines, respectively. (c) EOSs of Fe and ${\mathrm{XeFe}}_3/\mathrm{Xe}({\mathrm{Fe}}_{0.93}{\mathrm{Ni}}_{0.07}{)}_3$ as determined experimentally (dashed curves and solid symbols) and theoretically (solid curves) in this study. The volume of the superposition of $(\mathrm{Xe}+3\mathrm{Fe})/4$ is also shown for comparison. The x-ray wavelength is $0.31\AA$.

Figure 2

Calculated enthalpies of formation $\mathrm{\Delta }{H}^f$ of (a) ${\mathrm{XeFe}}_3$ and (b) ${\mathrm{XeNi}}_3$ with respect to the mixture of elemental $\mathrm{Xe}+3\mathrm{Ni}$ and $\mathrm{Xe}+3\mathrm{Fe}$, respectively. The enthalpies of the fcc structure in both compounds were calculated in an ordered structure. The two Pmmn structures in ${\mathrm{XeFe}}_3$ and ${\mathrm{XeNi}}_3$ are distinctly different and therefore distinguished as Pmmn (1) and Pmmn (2). (c) Schematic representations of the corresponding structures of ${\mathrm{XeFe}}_3$ and ${\mathrm{XeNi}}_3$.

Figure 3

(a) XRD patterns of a Xe-Ni mixture at 155 GPa as a function of temperature. The peak marked by the asterisk corresponds to the strongest peak of rhenium. The peaks of the hcp-Xe and fcc-Ni at RT before laser heating are marked with black and red vertical ticks, respectively. The vertical arrows mark the position of the Bragg peaks of the ${\mathrm{XeNi}}_3$ compound. The corresponding Miller indices for the fcc-Ni and the $\mathrm{f}\mathrm{c}\mathrm{c}\text{−}{\mathrm{XeNi}}_3$ are noted before and after laser heating, respectively. (b) XRD pattern of the synthesized ${\mathrm{XeNi}}_3$ compound in comparison to the calculated patterns of ordered (blue) and disordered (red) fcc crystal structures. The calculated pattern of the hcp-Xe is also shown for comparison. The vertical arrows mark the position of the additional Bragg peaks expected in an ordered ${\mathrm{Cu}}_3\mathrm{Fe}$-type fcc structure. The x-ray wavelength is 0.310 Å.

Figure 4

Calculated deformation charge density of ${\mathrm{XeFe}}_3$ in the (010) plane (a) and ${\mathrm{XeNi}}_3$ in the (001) plane (b) at 200 GPa.