Crystal Structure and Thermoelectric Properties of Layered Van der Waals Semimetal ZrTiSe4

Semiconductors have been studied as the most promising thermoelectric materials for the past decades due to their high Seebeck coefficient. Semimetals usually have a small Seebeck coefficient due to a small difference in the density of states between electrons and holes, compensating each other in the longitudinal direction. However, recent studies suggested that semimetals with large asymmetry between electrons and holes could be good thermoelectric candidates. Here, we report the crystal structure and thermoelectric properties of layered van der Waals semimetal ZrTiSe4. Micrometer-sized single crystals of ZrTiSe4 were obtained by the solid-state reaction. The single-crystal X-ray diffraction study suggests that this material exhibits a trigonal structure with space group P3m1 (no. 164), instead of a P2/m (no. 10) monoclinic phase reported in the previous work. ZrTiSe4 has the same crystal structure as ZrSe(2 )and TiSe2 with Zr and Ti atoms occupying the same site with equal occupancy. Polycrystalline ZrTiSe4 samples consolidated by cold pressing show a semi-metallic type of resistivity in the temperature range of 2-400 K and an n-type conducting behavior. The samples exhibit an unusually large Seebeck coefficient of -202 +/- 11 mu V K-1 at 300 K, notably higher than the parent phases of ZrSe2 and TiSe2 as well as other semimetals. Based on the first-principles calculations of the electronic band structure, the large Seebeck coefficient of ZrTiSe4 could arise from the conduction band convergence at the M point, resulting in a large density of states (DOS) effective mass for electrons and a highly asymmetric DOS about the chemical potential. Furthermore, owing to a low sound velocity and strong phonon scattering by boundaries and defects, the thermal conductivity exhibits weak temperature dependence and a low value of 2.2 +/- 0.4 W m(-1) K-1 at 300 K. This work provides useful insights into the crystal structure and thermoelectric properties of semimetal ZrTiSe4.