Electron Density Optimization and the Anisotropic Thermoelectric Properties of Ti Self-Intercalated Ti_1+xS₂ Compounds

Polycrystalline Ti1+xS2 (0.111 ≤ x ≤ 0.161) with high density and controllable composition were successfully prepared using solid-state reaction combined with plasma-activated sintering. Ti1+xS2 showed strong (00l) preferred orientation with Lotgering factor of 0.32–0.60 perpendicular to the pressing direction (⊥), whereas the preferred orientation was not obvious along the pressing direction (∥). This structural anisotropy resulted in distinct anisotropic thermoelectric transport properties in Ti1+xS2. At 300 K, while the Seebeck coefficient was weak anisotropic, the power factor and lattice thermal conductivity of Ti1+xS2 was much larger in the perpendicular direction as compared to that of the parallel direction, with an anisotropic ratio of 1.8–2.7 and 1.3–1.7, respectively. Theoretical calculations of formation energy of defects suggested that the excess Ti was most probably intercalated into the van der Waals gaps in metal-rich Ti1+xS2, consistent with X-ray diffraction, high-resolution transmission electron microscopy characterization and transport measurements. With increasing x, the carrier concentration and power factor of Ti1+xS2 dramatically increased because of the donor behavior of Ti interstitials, which was accompanied by a significant decrease in the lattice thermal conductivity owing to the strengthened phonon scattering from structural disorder. Because of its strongest (00l) preferred orientation and largest carrier mobility among all samples, Ti1.112S2 had the highest power factor of 22 μW cm–1 K–2 at 350 K perpendicular to the pressing direction, close to the value (37.1 μW cm–1 K–2) achieved in single-crystal TiS2. We found out that the maximum power factor and dimensionless figure of merit ZT could be achieved at an optimum carrier concentration of about 5.0 × 1020 cm–3. Finally, Ti1.142S2 acquired the highest ZT value of 0.40 at 725 K perpendicular to the pressing direction because of the beneficial preferred orientation, improved power factor, and reduced lattice thermal conductivity.