Lattice thermal resistivity and two valence band model a new understanding of transport properties of p-(Bi1-xSbx )2Te3

Usually (Bi_1-xSb_x)₂Te₃ is nowadays understood as a material with a well-known transport behaviour. This may hold true to a certain extent for optimised material at room temperature. In contrast, different conditions occur for material dedicated to low temperature cooling (100-200 K), as well as above room temperature, when thermal excitation of carriers becomes relevant. Both for high and low T transports, the band structure near the gap is of importance. From former experiments at He temperature a small valence band splitting was concluded. But attempts to interpret the anisotropic charge transport in a two-band model failed. Therefore, the single-band model established itself as a standard. Nevertheless, there are some questions which find no explanation within this picture. Detailed investigation concerning the lattice thermal resistivity did not yield the expected nearly linear temperature dependence. Instead, from experimental data a right-hand curvature is observed, predominantly at lower temperature. In the 200-300 K region a weak oscillation is superimposed to the monotonously increasing curve. These peculiarities are understandable as artefacts due to the single-band evaluation. Within a two-band model a nearly linear lattice thermal resistivity is obtained assuming an increased Lorenz number L due to unipolar diffusion. Furthermore, an oscillation of L due to non-equivalent intervalley scattering is evident, in good agreement to the calculated T dependence of the Fermi level. Conclusively, improved values of the lattice thermal resistivity are given. Thus, an estimation of the maximum figure of merit can be deduced yielding higher values in the 100-200 K region than previously calculated