Structural Evolution of the Binary System Ba-Si under High-pressure and High-temperature Conditions

A new silicon-rich binary compound BaSi₆ has been prepared by the treatment of the Ba₈Si₄₆ clathrate compound under a pressure of 15 GPa at 1000 °C, or from a stoichiometric mixture of BaSi₂ and Si by treatment under similar high-pressure and high-temperature conditions. The Rietveld refinements revealed that BaSi₆ is isomorphous with EuGa₂Ge₄, and crystallizes with space group Cmcm and the lattice parameters a = 4.485(1), b = 10.375(2), and c = 11.969(3) A . Each Ba atom is surrounded by 18 Si atoms in an irregularly shaped polyhedron @Si₁₈. The polyhedra are connected by sharing faces to form Ba containing tunnels along the a axis. All of the Si-rich compounds so far with atomic ratios Si/Ba > 2 in the binary system have been prepared only under high-pressure and high-temperature conditions. There is a general tendency that the Si/Ba ratio of the compounds increases with an increase of the pressure in the preparation.