Structural investigation of GeSb₆Te₁₀ and GeBi ₆Te₁₀ intermetallic compounds in the chalcogenide homologous series

The crystal structures of GeSb₆Te₁₀ and GeBi ₆Te₁₀ were scrutinized using an X-ray powder diffraction method, which revealed that these compounds crystallize in trigonally distorted cubic close-packed structures with a 51-layer period (). Each layer consists of a triangular atomic net; Te atoms occupy their own specific layers, whereas Ge, Sb and Bi atoms are located in the other layers. In these pseudobinary compounds, random atomic occupations of Ge and Sb/Bi are observed and the layers form two kinds of elemental structural blocks by their successive stacking along the c axis. These compounds can be presumed to be isostructural. It is known that the chemical formula of the chalcogenide compounds with the homologous structures found in these pseudobinary systems can be written as (GeTe)_n(Sb₂Te₃)_m or (GeTe) n (Bi2Te3) m (n, m: integer); the GeSb₆Te₁₀ and GeBi6Te10 investigated in this study, which correspond to the case in which n = 1 and m ×3, naturally have 3×l = 51-layer structures according to a formation rule l = 2n+5m commonly found in the compounds of these chalcogenide systems (l represents the number of layers in the basic structural unit). Calculations based on the density functional theory revealed that these materials are compound semiconductors with very narrow band gaps.