Crystal Structure and Physical Properties of Ternary Phases around the Composition Cu5Sn2Se7 with Tetrahedral Coordination of Atoms

A new monoclinic selenide Cu5Sn2Se7 was synthesized, and its crystal and electronic structure as well as thermoelectric properties were studied. The crystal structure of Cu5Sn2Se7 was determined by electron diffraction tomography and refined by full-profile techniques using synchrotron X-ray powder diffraction data: space group C2, a = 12.6509(3) Å, b = 5.6642(2) Å, c = 8.9319(4) Å, β = 98125(4)°, Z = 2; T = 295 K. Thermal analysis and high-temperature synchrotron X-ray diffraction indicated the decomposition of Cu5Sn2Se7 at 800 K with formation of the tetragonal high-temperature phase Cu4.90(4)Sn2.10(4)Se7: space group I4̅2m, a = 5.74738(1) Å, c = 11.45583(3) Å; T = 873 K. Both crystal structures are superstructures to the sphalerite type with tetrahedral coordination of the atoms. In agreement with chemical bonding analysis and band structure calculations, Cu5Sn2Se7 exhibits metal-like electronic transport behavior.