Synthesis, crystal and electronic structures, and physical properties of caged ternary Cu-rich antimonide: BaCu_7.31(3)Sb₅

A new caged Cu-rich antimonide, BaCu_7.31(3)Sb₅, was obtained from a direct combination of the elements in a graphite crucible under a high vacuum by a solid state reaction, and the structure was determined by the single-crystal X-ray diffraction method to be hexagonal P6₃/mmc (No.194), with a = 7.0154(4) Å, c = 12.5423(14) Å, V = 534.58(7) ų, and Z = 2. BaCu_7.31(3)Sb₅ is the first antimonide member of the BaNi₉P₅-type barium copper pnictides with a Cu2 site occupancy of 43.7(9)%, and the structure building unit is a 30-vertex Cu₁₈Sb₁₂ cage centered by a Ba atom. The Cu₁₈Sb₁₂ cages form chains along the c axis by sharing the opposite hexagonal (Cu2)₃(Sb2)₃ faces. Such a cage chain shares (Cu1)₂(Sb1)₂ rhomboidal faces with six neighboring chains along the [100], [010], and [110] directions to generate a 3D condensed metallic network. The electronic structure calculations by CASTEP indicate the metallic nature, which matches well with the metallic electrical conductivity, small Seebeck coefficient, and Pauli paramagnetism. The calculated formation energies indicate that BaCu_7.5Sb₅ -Ba₂Cu ₁₅Sb₁₀ with the Cu2 site half occupied is the energetically favorable stoichiometry compared with Ba₂Cu ₁₂Sb₁₀ (empty Cu2 site) and Ba₂Cu ₁₈Sb₁₀ (fully occupied Cu2 site).