Stability of xenon-sodium compounds at moderately low pressures

A growing body of theoretical and experimental evidence suggests that inert gases (He, Ne, Ar, Kr, Xe, Rn) become less and less inert under increasing pressure. Here we use crystal structure prediction software based on the ab initio evolutionary algorithm, named uspex, to predict stable compounds of Xe and Na at pressures below 100 GPa, and we find three stable compounds, NaXe, ${\mathrm{NaXe}}_3$, and ${\mathrm{NaXe}}_4$. The NaXe belongs to a well-known cubic CsCl structure type. ${\mathrm{NaXe}}_4$'s structure is common in amphiboles, whereas ${\mathrm{NaXe}}_3$ has a unique structure, analogous to the “post-perovskite” orthorhombic ${\mathrm{CaIrO}}_3$-type structure with Ir atoms removed. NaXe, ${\mathrm{NaXe}}_3$, and ${\mathrm{NaXe}}_4$ are found to be metallic.

Figure 1

Phase stability in the Na-Xe system. (a) Phase diagram of the Na-Xe system at a pressure of 0–100 GPa at a temperature of 0 K. (b) Convex hulls at pressures of 50, 70, and 100 GPa.

Figure 2

Crystal structures of Na-Xe compounds. (a) $pm\overline{3}m$ -NaXe: the center of the cube is Na, and the eight atoms around it are Xe. (b) $C2/m\text{−}{\mathrm{NaXe}}_4$: the blue ball is Xe and the yellow ball is Na. (c) Cmcm-${\mathrm{NaXe}}_3$: the blue ball is Xe and the yellow ball is Na. (d) ${\mathrm{CaIrO}}_3$ (post-perovskite phase), in which the purple, red, and cyan spheres are Ca, O, and Ir atoms, respectively.

Figure 3

Band structure of the structures. (a) Band structure of $pm\overline{3}m$-NaXe at a pressure of 43 GPa. (b) Band structure of Cmcm-${\mathrm{NaXe}}_3$ at a pressure of 63 GPa. (c) Band structure of $C2/m\text{−}{\mathrm{NaXe}}_4$ at a pressure of 82 GPa. The energies have been aligned to place the Fermi level ($E_{\mathrm{F}}$) at 0 eV.

Figure 4

Density of states of the structures. (a) Density of states of $pm\overline{3}m$-NaXe at 43 GPa. (b) Density of states of Cmcm-${\mathrm{NaXe}}_3$ at 63 GPa. (c) Density of states of Cmcm-${\mathrm{NaXe}}_4$ at 82 GPa. The density of states of elemental Xe under corresponding pressure is also added. The energies have been aligned to place the Fermi level ($E_{\mathrm{F}}$) at 0 eV.

Figure 5

Partial charge density in the energy range of $[E_{\mathrm{F}}\text{−}2\mathrm{eV},E_{\mathrm{F}}]$ of three structures. The charge density is expressed in $e$/$\mathrm{boh}{\mathrm{r}}^3$. The sections of $pm\overline{3}m$-NaXe (43 GPa) and Cmcm-${\mathrm{NaXe}}_3$ (63 GPa) are (1 1 0), and the section of $C2/m\text{−}{\mathrm{NaXe}}_4$ (82 GPa) is (–1 1 0).

Figure 6

(a) Crystal structure and (b) the 2D ELF corresponding to the slice plane of ${\mathrm{Na}}_9\mathrm{Xe}$ at 100 GPa. (c) Crystal structure and (d) the 2D ELF corresponding to the slice plane of ${\mathrm{Na}}_3{\mathrm{Xe}}_7$ at 100 GPa.