Crystallographic Constitution and the Thermoelectric Properties of Mixed Layered Tetradymite-like Ternary Compounds

The existence of the n-type nPbTemiddotmBi₂Te₃ and p-type nPbTemiddotmSb₂Te₃ homologous series compounds is found in the PbTe-Bi₂Te₃ and PbTe-Sb₂Te₃ systems by X-ray diffraction. The structures of the ternary compounds are formed by multilayer packets alternating orderly along a hexagonal "c" axis. In these layered structures, the bonding within the multi-layer packets has ionic-covalent character, while bonds between the packets are preferentially achieved by weak van der Waals forces. Such difference in the character of chemical bonding stipulates the anisotropy of crystal lattice and strongly marked cleavage planes. The anisotropy in thermoelectric properties has been studied in the n-type PbBi₄Te₇ and p-type PbSb₂Te₄ single crystals grown by Czochralski technique. The considerable anisotropy in thermoelectric properties (especially in PbSb₂Te₄) is discovered in the crystals by their measurement parallel and perpendicular to a hexagonal "c" axis. The lattice thermal conductivity measured parallel to "c" axis (kappa₃₃) is much smaller than that measured in perpendicular to "c" axis direction (kappa₁₁). Apparently, it is related to effective scattering of phonons by the potential barriers at the boundaries of the slabs, separated by van der Waals gaps