Synthesis and transport properties of the Te-substituted homologous compounds Pb5Bi6Se14−xTex (0 ≤ x ≤ 1.0)

The crystal structure and transport properties (2-723 K) of the homologous compound Pb5Bi6Se14 with partial substitution of Te for Se are studied by means of powder X-ray diffraction, scanning electron microscopy, electrical resistivity, thermopower, thermal conductivity and Hall effect measurements. Polycrystalline samples of Pb5Bi6Se14-xTex (0 ≤ x ≤ 1.0) were prepared by a two-step synthesis method based on the pseudo-binary PbSe-Bi2Se3 phase diagram combined with Te substitution in the PbSe precursor. The successful insertion of Te into the crystal structure of Pb5Bi6Se14 was confirmed by powder X-ray diffraction and scanning electron microscopy. Transport property measurements indicate an increase in the heavily doped character of the transport with increasing the Te concentration. The extremely low lattice thermal conductivity values (0.3-0.4 W m-1 K-1 at 723 K) that approach the glassy limit at high temperatures are nearly independent of the chemical composition suggesting no influence on point-defect scattering mechanisms in the substituted compounds. Despite the inherent complexity of this system, the evolution of the electronic properties with x is well described by a simple single-parabolic band model. Because the increase in the power factor with increasing x is compensated by the concomitant increase in the electronic thermal conductivity, this substitution does not yield enhanced ZT values with respect to the pristine compound with a similar peak ZT value of 0.5 achieved at 723 K. Nevertheless, the simple synthetic method used in this study to insert a doping element opens new avenues for controlling the transport properties of the homologous series (PbSe)5(Bi2Se3)3m (m = 1, 2 and 3).