Low phonon thermal conductivity of the layered Bi-Te intermetallic alloys

Good thermoelectric materials should have as low a thermal conductivity as possible in order to increase the figure of merit \textit{zT}=\textit{TS}$^{2}$/\textit{$\rho \kappa $}. The thermal conductivity of a material is often tuned using microstructure, chemistry, and/or crystal structure. For example, alloy disorder combined with high atomic weights plausibly lead to the low thermal conductivity of commercial Bi$_{2}$Te$_{3}$-based thermoelectrics. The crystal structure of the Bi-Te intermetallic phases is composed of layers of Bi and Bi$_{2}$Te$_{3}$ structural units, which can be varied nearly continuously by adjusting the Bi/Te ratio. Compared to both Bi$_{2}$Te$_{3}$ and elemental Bi, we found that these materials have a strongly reduced thermal conductivity below room temperature. However, this reduction did not depend on the Bi/Te ratio. The Debye-Calloway model was used to explore the origin of the low thermal conductivity in these materials.