Prediction of pressure-induced stabilization of noble-gas-atom compounds with alkali oxides and alkali sulfides

The cubic antifluorite structure comprises a fcc sublattice of anions with cations on the tetrahedral sites. The voids in the antifluorite structure that are crucial for superionicity in ${\mathrm{Li}}_2\mathrm{O}$ might also act as atomic traps. Trapping of guest atoms and small molecules within voids of a host structure leads to the formation of what are known as clathrate compounds. Here we investigate the possibility of trapping helium or larger neon guest atoms under pressure within alkali-metal oxide and sulfide structures. We find stable helium and neon-bearing compounds at very low pressures. These structures are stabilized by a reduction in volume from incorporation of helium or neon atoms within the antifluorite structure. We predict that ${\mathrm{NeCs}}_2\mathrm{S}$ could be stable at ambient pressure. Our study suggests a novel class of alkali oxide and sulfide materials incorporating noble-gas atoms that might potentially be useful for gas storage.

Figure 1

Crystal structure of antifluorite alkali-metal oxide with and without helium atoms. (a) The antifluorite structure of ${\mathrm{Na}}_2\mathrm{O}$. The Na atoms are shown in purple, and oxygen atoms are in red. The Na atoms occupy tetrahedral positions, and one of the Na atoms is shown at the center of its green tetrahedron. (b) He atoms (light blue) are included within the octahedral voids, and one of the octahedrons is highlighted in purple with a He atom visible at its center.

Figure 2

Enthalpy-pressure relations for noble-gas compounds. (a)–(e) He in alkali-metal oxides, (f)–(j) He in alkali-metal sulfides, and (k)–(o) Ne in alkali-metal sulfides. The enthalpies are given with respect to those of the alkali oxides/sulfides with included noble-gas atoms.

Figure 3

Enthalpy-pressure relations for noble-gas compounds calculated with the PBE, PBEsol and PBE+DFT-D3 (labeled as PBE-D3) functionals.

Figure 4

Electronic band structure and density of states of ${\mathrm{Na}}_2\mathrm{O}$ (red dashed line) and ${\mathrm{HeNa}}_2\mathrm{O}$ (blue solid line) at 20 GPa. The zero of energy is at the valence band maximum. The He $1s$ band lies at about $-13.5$ eV.

Figure 5

(a) Phonon band structures and densities of states of ${\mathrm{K}}_2\mathrm{S}$ (red dashed line) and ${\mathrm{NeK}}_2\mathrm{S}$ (blue solid line) at 5 GPa. The Ne-derived vibrations give Einstein-oscillator-like phonon bands at about 110 ${\mathrm{cm}}^{-1}$ which show some hybridization with the ${\mathrm{K}}_2\mathrm{S}$-derived bands. (b) The localized modes obtained for Ne at the $\mathrm{\Gamma }$ point. (c) and (d) The localized modes with the highest frequency at the $\mathrm{\Gamma }$ point. The vectors in (b), (c), and (d) show the polarization directions and amplitudes of the atoms. Ne, K, and S atoms are colored magenta, purple, and yellow, respectively.