Crystal Structures and Thermoelectric Properties of Layered Compounds in the ATe-Bi2Te3 (A = Ge, Sn, Pb) Systems

The search for new chalogenide thermoelectric materials with complex crystal structures has led many researchers to focus on tetradymite-like layered ternary compounds. Single-crystal and powder x-ray diffraction studies of compounds in the nATe .mBi2Te3 (A = Ge, Sn, Pb) homologous series in the pseudobinary systems ATe-Bi2Te3 reveal a marked dissimilarity of the GeTeBi2Te3 system from SnTe-Bi2Te3 and PbTe-Bi2Te3. The GeTe-Bi2Te3 system contains, along with GeBi2Te4 (n/m = 1), GeTe-rich (n/m > 1) and Bi2Te3-rich (n/m < 1) members of the nGeTe mBi2Te3 series. In contrast, the Sn and Pb systems contain only Bi2Te3-rich compounds (n/m < 1). This dissimilarity is interpreted in terms of the crystal structures of GeTe, SnTe, and PbTe. The thermoelectric properties of nPbTe DT mBi2Te3 compounds are studied in a broad temperature range. The thermoelectric figures of merit of unoptimized n-type PbBi4Te7 and PbBi2Te4 are ZT = 0.5 and 0.4 at 600 and 650 K, respectively. Comparison of the thermoelectric properties of the isostructural 12-layer compounds GeBi4Te7, SnBi4Te7, and PbBi4Te7 indicates that the maximum in thermopower shifts to higher temperatures in going from the Ge to Sn and Pb compounds.