Sb and Se substitution in CsBi ₄Te ₆: The semiconductors CsM ₄Q ₆ (M = Bi, Sb; Q = Te, Se), Cs ₂Bi ₁₀Q ₁₅, and CsBi ₅Q ₈

The solid solutions of CsBi ₄Te ₆, a high ZT material at a low temperature region, with Sb and Se were synthesized with general formulas CsBi _4-xSb _xTe ₆ and CsBi ₄Te _6-ySe _y. The introduction of Sb and Se in the lattice of CsBi ₄Te ₆ 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 CsBi ₄Te ₆ 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 < 2.4 with the formula of CsBi ₅Te _7.5-ySe _y (or Cs ₂Bi ₁₀Q ₁₅, (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 CsBi ₄Te ₆ (23 Å). The CsBi ₅Te _7.5-ySe _y possesses Bi/Te slabs that extend by an additional "Bi ₂Te ₃" unit compared to the structure of CsBi ₄Te ₆, 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 CsBi ₅Te _3.6Se _4.4 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 CsBi ₄Te ₆.