Rare Helium-Bearing Compound ${\mathrm{FeO}}_2\mathrm{He}$ Stabilized at Deep-Earth Conditions

There is compelling geochemical evidence for primordial helium trapped in Earth’s lower mantle, but the origin and nature of the helium source remain elusive due to scarce knowledge on viable helium-bearing compounds that are extremely rare. Here we explore materials physics underlying this prominent challenge. Our structure searches in conjunction with first-principles energetic and thermodynamic calculations uncover a remarkable helium-bearing compound ${\mathrm{FeO}}_2\mathrm{He}$ at high pressure-temperature conditions relevant to the core-mantle boundary. Calculated sound velocities consistent with seismic data validate ${\mathrm{FeO}}_2\mathrm{He}$ as a feasible constituent in ultralow velocity zones at the lowermost mantle. These mutually corroborating findings establish the first and hitherto only helium-bearing compound viable at pertinent geophysical conditions, thus providing vital physics mechanisms and materials insights for elucidating the enigmatic helium reservoir in deep Earth.

Figure 1

(a) Crystal structure of the predicted $Fm\text{−}3m$ ${\mathrm{FeO}}_2\mathrm{He}$, where the golden, red, and white spheres represent Fe, O, and He atoms, respectively. (b) Calculated formation enthalpy of $Fm\text{−}3m$ ${\mathrm{FeO}}_2\mathrm{He}$ relative to decomposition products ${\mathrm{FeO}}_2+\mathrm{He}$ as a function of pressure at 0 K. (c) The convex hull (black solid lines) of the ${\mathrm{FeO}}_2-\mathrm{He}$ system at 135 GPa, where the enthalpy values for structures with partial He content ${\mathrm{FeO}}_2{\mathrm{He}}_x$ with $x<1$, as indicated by the numbers at the top of the panel, are connected by black dashed lines. (d) Calculated phonon dispersion of $Fm\text{−}3m$ ${\mathrm{FeO}}_2\mathrm{He}$ at 135 (black lines) and 300 GPa (red lines).

Figure 2

(a) Pressure-temperature ($P\text{−}T$) phase diagram showing the stability field of $Fm\text{−}3m$ ${\mathrm{FeO}}_2\mathrm{He}$ and its boundary with the decomposition products ${\mathrm{FeO}}_2+\mathrm{He}$. Also shown is the geotherm indicating the $P\text{−}T$ profile in Earth’s interior. (b) Calculated mean-square-deviation (MSD) of the atomic positions of $Fm\text{−}3m$ ${\mathrm{FeO}}_2\mathrm{He}$ at 135 GPa and 3000 K.

Figure 3

Electronic band structures (left) and density of states (DOS) (right) of $Fm\text{−}3m$ ${\mathrm{FeO}}_2\mathrm{He}$ at indicated pressures.

Figure 4

Stereographic projections of the calculated $P$-wave velocity $V_P$ (in km/s) and $S$-wave anisotropy $A{V}_S$ (in %) for $Fm\text{−}3m$ ${\mathrm{FeO}}_2\mathrm{He}$ at 135 GPa and 3000 K. The coordinate axes are $X_1=[100]$, $X_2=[010]$, $X_3=[001]$. The black square and white circle in each plot indicate the crystallographic directions of the maximum and minimum values, respectively.