Small-polaron transport and thermoelectric properties of the misfit-layer composite (BiSe)$_{109}$TaSe$_{2}/$TaSe$_{2}$

We studied the thermoelectric properties of the composite of misfit-layered compounds (BiSe)$_{109}$TaSe$_{2}$ and TaSe$_{2}$. The x-ray diffraction pattern on the cross-sectional plane of the sintered body shows a preferred orientation of the (00$l)$ direction for (BiSe)$_{109}$TaSe$_{2}/$TaSe$_{2}$ indicating anisotropic alignment during hot pressing. Because of the crystallographic alignment, the temperature-dependent electrical resistivity $\rho (T )$, Seebeck coefficient $S(T )$, and the thermal conductivity $\kappa (T)$ exhibit in-plane and out-of-plane anisotropic transport behavior. The Seebeck coefficient is very low because of the coexistence of electron and hole mixing, as confirmed by the two-carrier model. The lattice thermal conductivity $\kappa_{L} $of the covalent bonding layer (in-plane) is lower than those of the layer with van der Waals bonding (out-of-plane) implying the existence of a charge density wave along the in-plane. We observed a sign anomaly of the positive Hall coefficient $R_{H}$ and negative Seebeck coefficient $S$. According to Holstein's small-polaron model, the sign anomaly may come from the odd number of small-polaron hopping sites.