Crystal structure and thermoelectric properties of the mixed layered compounds of the (GeTe)m(Bi2Te3)m homologous series

The initial members of the (GeTe)_n(Bi₂Te₃)_m homologous series (Ge₃Bi₂Te₆, GeBi₂Te ₄ and GeBi₄Te₇ compounds) have been investigated by X-ray diffraction study. A high degree of lattice disorder is found in the melt-grown compounds. The dependences of the a and c/N parameters of the hexagonal lattices (N-number of layers per unit cell) on the cation-to-anion ratio have been analyzed for nGeTe.mBi ₂Te₃ compounds. These dependences were considered to be very dose to linear. Based on these results, the lattice parameters were evaluated for another member of the homologous series for which experimental data are absent. A strong dependence of thermoelectric properties at 300 K on deviation from stoichiometry has been found for three above-mentioned compounds. All three compounds are characterized by extremely low lattice thermal conductivity which is probably related to the high degree of disorder in the cation sublattice as well as in the anion one