Linear, Hypervalent Se Units and Unprecedented CuSe Building Blocks in the Copper(I) Selenide BaCuSe.

Single-crystal and polycrystalline BaCuSe 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 BaCuSe. The title compound can be rationalized as (Ba)(Cu)(Se)(Se)(Se) 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 Å). It contains unprecedented CuSe 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 Se units, resulting in "sandwichlike", layered building blocks, which are stacked along c and separated by Ba. BaCuSe is the first example where Se and Se groups coexist. We were able to visualize the crystal structure by recording HAADF images, which clearly reveal the CuSe fragments and linear Se units. The title compound is a charge-balanced semiconductor and possesses a large Seebeck coefficient (380 μV K at 200 K) and a low thermal conductivity (0.77 W m K at 200 K)-two requirements for efficient thermoelectric materials.