He-Mg compounds and helium-driven nonmetal transition in metallic magnesium

The polymorphism and mechanism of helium compounds are crucial for understanding the physical and chemical nature of He-bearing materials under pressures. Here, we predict two types of He-bearing compounds, MgHe and $\mathrm{M}{\mathrm{g}}_{n}\mathrm{He}$ $(n=6, 8, 10, 15, \text{and} 18)$, being formed >750 GPa, by unbiased ab initio structure search. An unexpected band gap is opened in MgHe at as low as \ensuremath{\sim}200 GPa. This is a case of noble gas--driven metal-nonmetal transition. The same mechanism is demonstrated as also being applicable to other metallic elements and making beryllium transform into a nonmetallic state, an achievement that is impossible otherwise. Furthermore, the stability of the simple cubic phase of Mg (Mg-sc) is greatly enhanced by mixing with He, which lowers the critical pressure of pure Mg-sc from \ensuremath{\sim}1.1 TPa down to 750 GPa to form ordered substitutional alloying phase of $\mathrm{M}{\mathrm{g}}_{n}\mathrm{He}$ on a simple cubic lattice of Mg. This is a report on a Mg-based noble gas--substitutional alloy, in sharp contrast with the conventional wisdom that He prefers interstitial sites. The observed striking influences of He demonstrate the rich physics and chemistry of He-bearing compounds under ultrahigh pressures.