Single crystal structure elucidation and thermoelectric properties of a long-periodically ordered germanium arsenic telluride

Single-crystal X-ray diffraction revealed that the structure of As2Ge7Te10 = (GeTe)7As2Te3 (space group R 3¯m) consists of rocksalt-type building blocks separated by van der Waals gaps. As and Ge are disordered on some of the cation sites. The rhombohedral 57R stacking sequence results in an extraordinarily large lattice parameter of c = 103.41 (2) Å. When such structures are formed, the extremely long diffusion pathways required to obtain completely ordered compounds often lead to samples that contain intergrown rocksalt-type slabs with varying thicknesses. As a result, the system (GeTe)nAs2Te3 is prone to form both disordered and inhomogeneous samples, expecially for n > 9. Yet, powder diffraction data reveal that such samples contain 57R-As2Ge7Te10 as a main component. Such inhomogeneous materials exhibit promising thermoelectric properties: the Seebeck coefficient as well as the lattice part of the thermal conductivity are in the same range as germanium antimony tellurides with high figures of merit ZT. However, the high electrical conductivity and consequently the rather dominant electronic component of thermal transport still impede such ZT values for germanium arsenic tellurides.