Sb and Se Substitution in CsBi4Te6: The Semiconductors CsM4Q6 (M = Bi, Sb; Q = Te, Se), Cs2Bi10Q15, and CsBi5Q8

The solid solutions of CsBi4Te6, a high ZT material at a low temperature region, with Sb and Se were synthesized with general formulas CsBi4‑xSbxTe6 and CsBi4Te6‑ySey. The introduction of Sb and Se in the lattice of CsBi4Te6 is possible but only to a limited extent. The Sb and Se atoms substituted are not uniformly distributed over all crystallographic sites but display particular site preferences. The structure of new Sb/Bi solid solutions retains the original framework of CsBi4Te6 composed of NaCl-type Bi/Te slabs interconnected by characteristic Bi–Bi bonds and Cs atoms located in the interlayer space. A structurally modified phase in Se/Te solid solutions was found from the reactions targeted for 0.2 y 5Te7.5‑ySey (or Cs2Bi10Q15, (Q = Se, Te)). The new structure is constructed by the same structural motif with an extended Bi/Te slab (29 Å) compared to that in CsBi4Te6 (23 Å). The CsBi5Te7.5‑ySey possesses Bi/Te slabs that extend by an additional “Bi2Te3” unit compared to the structure of CsBi4Te6, which implies the existence of a phase homology of compounds with the adjustable parameter being the width of the Bi/Q slab. In the reactions targeted for the compounds with higher y, a new phase CsBi5Te3.6Se4.4 with a different type of framework was found. The electrical conductivity and thermopower for the selected samples show p-type conduction with metallic behavior. The room temperature values measured are in the range of 300–1100 S/cm and 100–150 μV/K for Sb-substituted samples and 20–500 S/cm and 70–140 μV/K for Se-substituted samples, respectively. Thermal conductivities of these samples are in the range of 0.9–1.2 W/m·K at room temperature. Tailoring the transport behavior of these materials for thermoelectric applications may be achieved by doping, as is possible for the parent compound CsBi4Te6.