Pressure-Induced Stable Binary Compounds of Magnesium and Germanium.

Abstract

Motivated by the possibility to obtain unusual stoichiometric compounds (e.g., Na-Cl and Mg-O systems) with exotic properties at high pressures, we systematically investigated the high-pressure structures and chemical bonding of Mg-Ge systems by using a structure-searching method and first-principles calculations. Compared with the stable composition of Mg2 Ge at ambient pressure, several stoichiometries (e.g., Mg3 Ge, MgGe, and MgGe2 ) were predicted to be stable under high pressures. The Pm m Mg3 Ge structure consists of a 12-fold-coordinated face-sharing GeMg12 cuboctahedron, whereas the P4/mmm MgGe and Cmcm MgGe2 phases form MgGe8 hexahedrons and MgGe4 polygons, respectively. All the stable phases of Mg-Ge compounds under high pressures exhibit metallic features owing to overlap between the conduction and valence bands. For Cmcm MgGe2 , the projected density of states near the Fermi energy mainly derive from Ge s, Ge p, and Ge d, which are responsible for its metallicity. The calculated superconducting critical temperature values of Cmcm Mg2 Ge and P4/mmm MgGe reach 10.3 and 9.07 K at 5 GPa, respectively.