Pressure-induced stable structures and physical properties of Sr–Ge system

We have systematically investigated the structures of Sr–Ge system under pressures up to 200 GPa and found six stable stoichiometric structures, they being Sr ₃Ge, Sr ₂Ge, SrGe, SrGe ₂, SrGe ₃, and SrGe ₄. We demonstrate the interesting structure evolution behaviors in Sr–Ge system with the increase of germanium content, Ge atoms arranging into isolated anions in Sr ₃Ge, chains in Sr ₂Ge, square units in SrGe, trigonal units and hexahedrons in SrGe ₂, cages in SrGe ₃, hexagons and Ge ₈ rings in SrGe ₄. The structural diversity produces various manifestations of electronic structures, which is of benefit to electrical transportation. Among them, these novel phases with metallic structures show superconductivity (maximum T _c ∼ 8.94 K for Pmmn Sr ₃Ge). Notably, the n-type semiconducting Pnma SrGe ₂ structure exhibits high Seebeck coefficient and excellent electrical conductivity along the y direction, leading to a high ZT value up to 1.55 at 500 K, which can be potential candidates as high-performance thermoelectrics. Our results will enable the development of fundamental science in condensed matter physics and potential applications in novel electronics or thermoelectric materials.