Linear, Hypervalent Se₃^4- Units and Unprecedented Cu₄Se₉ Building Blocks in the Copper(I) Selenide Ba₄Cu₈Se₁₃

Single-crystal and polycrystalline Ba₄Cu₈Se₁₃ were synthesized; the average crystal structure was solved by single-crystal X-ray diffraction, and the structural model was confirmed by a detailed electron microscopy study of polycrystalline Ba₄Cu₈Se₁₃. The title compound can be rationalized as (Ba²⁺)₄(Cu⁺)₈(Se^2-)₂(Se₂^2-)₄(Se₃^4-) and crystallizes in a new structure type (space group C2/c with a = 9.171(8) Å, b = 9.146(8) Å, c = 27.35(3) Å, β = 93.21(3)°, and V = 2291 ų). It contains unprecedented Cu₄Se₉ fragments with planar Cu rectangles. These fragments form two-dimensional layers via regular (2c-2e) Se-Se bonds. Two of these layers are then connected in the third dimension via linear, hypervalent Se₃^4- units, resulting in "sandwichlike", layered building blocks, which are stacked along c and separated by Ba. Ba₄Cu₈Se₁₃ is the first example where Se₂^2- and Se₃^4- groups coexist. We were able to visualize the crystal structure by recording HAADF images, which clearly reveal the Cu₄Se₉ fragments and linear Se₃^4- units. The title compound is a charge-balanced semiconductor and possesses a large Seebeck coefficient (380 μV K^-1 at 200 K) and a low thermal conductivity (0.77 W m^-1 K^-1 at 200 K)-two requirements for efficient thermoelectric materials.