Thermoelectric properties of phosphorus-doped van der Waals crystal Ta4SiTe4

Abstract Recently, one-dimensional van der Waals crystal Ta4SiTe4 has been reported as a promising low-temperature thermoelectric material. Extraordinarily high power factor has been reported for the one-dimensional Ta4SiTe4 single whisker and two-dimensional (PVDF)/Ta4SiTe4 composite film, but the thermoelectric properties of three-dimensional Ta4SiTe4 polycrystalline bulks have been rarely investigated. In this study, we prepared a series of phosphorus (P)-doped Ta4SiTe4 polycrystalline bulks by using the high-temperature synthesis and cold press method. Their electrical and thermal transport properties along the directions parallel and perpendicular to the pressure were systematically investigated. Upon doping P at the Si-sites to increase the carrier concentration, the electrical conductivity is enhanced while the intrinsic excitation is suppressed, resulting in significantly improved power factor approaching the theoretically optimal value. All Ta4Si1-xPxTe4 bulks possess low lattice thermal conductivities with the values below 1.2 Wm-1K-1. The weak van der Waals interaction among the [Si2Ta8Te8]n chains results in the appearance of low-lying optical modes and introduces additional scattering to phonons. A peak thermoelectric figure of merit value of 0.18 is obtained at 300 K for polycrystalline Ta4Si0.995P0.005Te4, about double that of pristine polycrystalline Ta4SiTe4.