Crystal-structure searches predict that helium and neon, normally inert, form thermodynamically stable inclusion compounds when trapped in the voids of alkali oxide and alkali sulfide antifluorite hosts at pressures from ambient up to roughly 100 GPa, with stabilization enthalpies of only a few to hundreds of meV per formula unit. None of these predicted phases — for example HeNa2_2O, HeK2_2O, HeRb2_2O, HeK2_2S, HeRb2_2S, NeNa2_2S, NeK2_2S, NeRb2_2S, and NeCs2_2S, several of which are predicted to survive to ambient pressure — has ever been synthesized, and their predicted stability windows rest on semilocal density-functional theory with dispersion corrections fitted at ambient pressure. The open problem is to establish whether this predicted class of noble-gas inclusion compounds exists as real, characterizable materials: to synthesize and structurally confirm representative members in the laboratory, to determine their actual formation pressures and dynamical stability — including the flagged low-pressure dynamical instability of HeK2_2S — and their recoverability on decompression, using methods whose accuracy is commensurate with the small energetic margins involved (e.g. high-pressure diffraction, and beyond-DFT electronic-structure treatments including nuclear quantum effects).

Background

The alkali metal oxides and sulfides of composition A2_2X (A = Li, Na, K, Rb, Cs; X = O, S) mostly crystallize in the antifluorite structure: a face-centered cubic sublattice of anions (X) with alkali cations (A) occupying the tetrahedral sites, leaving octahedrally coordinated empty voids. If guest atoms are trapped in the voids of a host lattice without forming conventional chemical bonds, the result is an inclusion (or clathrate-type) compound. Alkali oxides and sulfides are classic ionic crystals used in catalysis, CO2_2 capture, gas detectors, and battery materials, and their voids had earlier been discussed in connection with superionicity in Li2_2O (a state in which one ionic species becomes mobile as in a liquid while the framework remains solid).

Helium and neon are the most chemically inert elements: until recently neither had any known stable solid compound. High-pressure physics has progressively eroded this inertness for helium, while neon still has no experimentally confirmed compound of any kind. Structure searching with the ab initio random structure searching (AIRSS) method — a technique that generates many random candidate crystal structures and relaxes them with density-functional theory (DFT) to find low-enthalpy phases — combined with DFT enthalpy calculations predicted, and diamond-anvil-cell (DAC) experiments then confirmed, the first stable helium compound Na2_2He above about 113 GPa (Dong et al., Nature Chemistry 2017), an electride-like phase in which helium forces electron localization in the interstitial sites rather than forming covalent bonds. The helium pressure medium in such experiments was thereby shown to be a potential reagent. Theory soon established a general mechanism (Liu et al., Nature Communications 2018): inserting He (or Ne) into ionic hosts with unequal numbers of cations and anions, such as MgF2_2 or MgO, can lower the long-range electrostatic (Madelung) energy of the crystal enough to compensate the cost of embedding the noble gas, so that a ternary inclusion phase becomes enthalpically favored under compression. Experimentally, helium insertion has now been demonstrated in open-framework A-site-vacant perovskites — hosts whose cages trap neutral atoms — including [He2_2][CaZr]F6_6 (Lloyd et al., 2021) and Hex_xScF3_3, which forms above about 1.5 GPa at room temperature with direct X-ray diffraction and gas uptake/release evidence (Ma et al., Chemistry of Materials 2025).

In this context Gao, Sun, Pickard and Needs (Phys. Rev. Materials 3, 015002 (2019); arXiv:1409.2227) used AIRSS and DFT to study He and Ne trapped in all ten antifluorite alkali oxides and sulfides Li2_2O through Cs2_2S. They predicted stable noble-gas-bearing phases at remarkably low pressures: HeNa2_2O over 13–105 GPa, HeK2_2O over 3–45 GPa, HeRb2_2O over 3.7–34.5 GPa, narrow sulfide windows HeK2_2S (1.3–2.4 GPa) and HeRb2_2S (0–2 GPa), and, with larger stabilization, the neon sulfides NeNa2_2S (8.8–13 GPa), NeK2_2S (0–7.5 GPa), NeRb2_2S (0–6 GPa) and NeCs2_2S (0–3.3 GPa); with dispersion-corrected functionals, NeK2_2S, NeRb2_2S and NeCs2_2S were predicted to be stable even at ambient pressure, raising the prospect of recoverable noble-gas storage materials. The physics is a volume-reduction effect: the noble gas occupies void space and allows the whole crystal to contract, paying its embedding cost. However, the paper itself flags three limitations that define the open problem. First, the stabilization enthalpies are small — after quasiharmonic (QHA, i.e. including thermal-expansion and phonon enthalpy) and zero-point-energy corrections, NeK2_2S is only about 54 meV per formula unit more stable than Ne plus K2_2S at 5 GPa, and HeNa2_2O only about 175 meV/f.u. at 40 GPa — margins comparable to the differences between DFT approximations. The dispersion corrections used (e.g. DFT-D3) are fitted at ambient pressure, semilocal functionals like PBE systematically underestimate band gaps, and DFT lattice constants for solid helium are about 10% too small because nuclear quantum effects (the quantum zero-point motion of the light He and Ne nuclei) are neglected; the authors state that more accurate approaches such as diffusion quantum Monte Carlo (DMC, a stochastic many-body method that can reach near-chemical accuracy for solids) could change the quantitative formation pressures. Second, the paper contains an internal tension about HeK2_2S: one passage reports no imaginary phonon frequencies (i.e. no dynamical instability) down to zero pressure, while another states that HeK2_2S is dynamically unstable at low pressures and stabilized only above a few GPa — dynamical stability meaning that the phonon spectrum contains no imaginary modes, so the lattice does not spontaneously distort. Third, and most importantly, none of the predicted compounds has been synthesized: whether they form in the laboratory, at what pressures, and whether any can be recovered on decompression remain untested.

Problem Statement

Determine whether the predicted class of pressure-stabilized noble-gas inclusion compounds — helium or neon atoms trapped in the voids of alkali oxide and alkali sulfide hosts, NgA2_2X with Ng = He or Ne and A = Li, Na, K, Rb, Cs; X = O or S — can be realized as actual materials, by (i) synthesizing and structurally confirming representative members of the predicted phases (HeNa2_2O, HeK2_2O, HeRb2_2O, HeK2_2S, HeRb2_2S, NeNa2_2S, NeK2_2S, NeRb2_2S, NeCs2_2S) under high pressure in the laboratory, identifying the crystal structures of the phases that form and the pressure ranges over which each inclusion compound is stable relative to the separated noble gas plus host phase; (ii) determining the true thermodynamic and dynamical stability windows of these compounds with quantitative methods whose accuracy is commensurate with the small stabilization enthalpies involved (a few to hundreds of meV per formula unit) — beyond semilocal DFT with ambient-fitted dispersion corrections, including treatments of nuclear quantum effects for the light He and Ne guests, e.g. by diffusion quantum Monte Carlo — and in particular resolving the internally contradictory low-pressure dynamical-stability picture of HeK2_2S; and (iii) establishing whether any of the phases, especially those predicted stable at or near ambient pressure (NeK2_2S, NeRb2_2S, NeCs2_2S), can be recovered on decompression and retained metastably for use as noble-gas storage materials. These three components serve one common objective: converting the DFT prediction that this class of noble-gas inclusion compounds exists into experimentally verified, quantitatively characterized materials with known stability ranges. All three components (i), (ii) and (iii) are required for a full resolution of the problem, and no one of them on its own resolves it. The nine phases enumerated under (i) are the candidate members of the predicted class rather than a mandatory complete target set: the class-level claim is established once one or more representative members have been synthesized and structurally confirmed and their quantitative stability windows and decompression behaviour determined, so that realizing every listed phase is not required.

References

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