Elemental Distribution and Thermoelectric Properties of Layered Tellurides 39 R ‐M 0.067 Sb 0.667 Te 0.266 (M=Ge, Sn)

The isostructural phases 39 R ‐Ge 0.067 Sb 0.667 Te 0.266 ( R $\bar 3$ m , a =4.2649(1), c =75.061(2) Å) and 39 R ‐Sn 0.067 Sb 0.667 Te 0.266 ( R $\bar 3$ m , a =4.2959(1), c =75.392(2) Å) were prepared by quenching stoichiometric melts of the pure elements and subsequent annealing at moderate temperatures. Their structures are comparable to “superlattices” synthesized by layer‐by‐layer deposition onto a substrate. These structures show no stacking disorder by electron microscopy. The structure of the metastable layered phases are similar to that of 39 R ‐Sb 10 Te 3 (equivalent to Sb 0.769 Te 0.231 ), which contains four A7 gray‐arsenic‐type layers of antimony alternating with Sb 2 Te 3 slabs. Joint refinements on single‐crystal diffraction data using synchrotron radiation at several K edges were performed to enhance the scattering contrast. These refinements show that the elemental distributions at some atom positions are disordered whereas otherwise the structures are long‐range ordered. The variation of the elemental concentration correlates with the variation in interatomic distance. Z‐contrast scanning transmission electron microscopy (HAADF‐STEM) on 39 R ‐Ge 0.067 Sb 0.667 Te 0.266 confirms the presence of concentration gradients. The carrier‐type of the isostructural metal (A7‐type lamellae)‐semiconductor heterostructures (Ge/Sn‐doped Sb 2 Te 3 slabs) varies from n‐type (Ge 0.067 Sb 0.667 Te 0.266 ) to p‐type (Sn 0.067 Sb 0.667 Te 0.266 ). Although the absolute values of the Seebeck coefficient reached about 50–70 μV/K and the electrical conductivity is relatively high, the two isotypic phases exhibit a maximal thermoelectric figure of merit ( ZT ) of 0.06 at 400 °C as their thermal conductivity (κ≈8–9.5 W/mK at 400 °C) lies interestingly in between that of antimony and pure Sb 2 Te 3 .