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

Abstract

The solid solutions of CsBi₄Te₆, a high ZT material at a low temperature region, with Sb and Se were synthesized with general formulas CsBi_4‑<i>x</i>Sb_<i>x</i>Te₆ and CsBi₄Te_6‑<i>y</i>Se_<i>y</i>. 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 < <i>y</i> < 2.4 with the formula of CsBi₅Te_{7.5‑<i>y</i>}Se_<i>y</i> (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 CsBi₅Te_{7.5‑<i>y</i>}Se_<i>y</i> 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 <i>y</i>, a new phase CsBi₅Te_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 CsBi₄Te₆