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

Abstract

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 <i>C</i>2, <i>a</i> = 12.6509(3) Å, <i>b</i> = 5.6642(2) Å, <i>c</i> = 8.9319(4) Å, β = 98125(4)°, <i>Z</i> = 2; <i>T</i> = 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 Cu_4.90(4)Sn_2.10(4)Se₇: space group <i>I</i>4̅2<i>m</i>, <i>a</i> = 5.74738(1) Å, <i>c</i> = 11.45583(3) Å; <i>T</i> = 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