Effects of Transition Metal Substitution on the Thermoelectric Properties of Metallic (BiS)_1.2(TiS₂)₂ Misfit Layer Sulfide

The misfit layer compounds (BiS)_1.2(TiS₂)₂, with a natural superlattice structures, are of substantial interest as thermoelectric materials. In this work we doped the Ti sites of (BiS)_1.2(TiS₂)₂ with a series of transition metal (TM) elements (V, Cr, Mn, Fe, Co, Ni, Cu, and Zn), to optimize its transport properties and thermoelectric performance. X-ray diffraction confirmed all the resulting compositions were single-phase. X-ray photoelectron spectroscopy revealed the valence states of the doping elements, indicating they behave as acceptors and reduce the carrier concentration; this was also apparent from Hall measurements. However, because of the non-parabolic band structure of (BiS)_1.2(TiS₂)₂, reduction of carrier concentration by doping with most of the TM elements did not improve the Seebeck coefficient. Exceptions were V and Cr. For these elements, the effective mass of electrons was maintained, or even enhanced, resulting in improvement of the Seebeck coefficient. Furthermore, the stacking disorder present in undoped (BiS)_1.2(TiS₂)₂ was not observed for the TM element-doped samples, resulting in increased lattice thermal conductivity. Although the power factor of these materials was not optimized, because of the large reduction in electronic thermal conductivity upon doping, the Cr-doped sample had a higher figure of merit than the undoped material.