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$_{3}$Ge, Sr$_{2}$Ge, 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$_{3}$Ge, chains in Sr$_{2}$Ge, 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 $T_{rm c}sim 8.94$ K for Pmmn Sr$_{3}$Ge). 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.