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

The polymorphism and mechanism of helium compounds is crucial for understanding the physical and chemical nature of He-bearing materials under pressures. Here, we predict two new types of He-bearing compounds, MgHe and MgnHe (n = 6, 8, 10, 15, 18), being formed above 750 GPa by unbiased ab initio structure search. An unexpected bandgap is opened up in MgHe at as low as around 200 GPa. This is the first case of noble gas driven metal-nonmetal transition in all elements. The same mechanism is demonstrated also being applicable to other metallic elements, and making beryllium transform into a non-metallic state, a triumph 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 about 1.1 TPa down to 750 GPa to form ordered substitutional alloying phase of MgnHe on a simple cubic lattice of Mg. This is the first report on Mg-based noble gas substitutional alloy, in sharp contrast to the conventional wisdom that He preferring interstitial sites. The observed striking influences of He demonstrate the rich physics and chemistry of He-bearing compounds under ultra-high pressures.