Oxygen Vacancy in Magnéli Phases and Its Effect on Thermoelectric Performances

Magnéli phases exhibit significant potential for applications in electronic materials in energy conversion due to their high electrical conductivity and excellent thermal stability. In this study, single-phase Ti_nO_2n-1 (n = 4, 5, 6) bulk materials were successfully prepared by a combination of the carbothermal reduction of nano-sized rutile TiO₂ and hot-press sintering methods. The relationships between the phase evolution, microstructural features, and thermoelectric performance were investigated systematically. Synchrotron X-ray diffraction (SXRD) and scanning electron microscopy (SEM) analyses revealed that the Ti₄O₇ and Ti₅O₉ materials had single-phase structures with high densities (relative density > 97%) and no obvious grain boundary holes or microcracks. We tested the thermoelectric properties of the Magnéli phases in the temperature range of 300-1100 K. The Magnéli phases exhibited a significant temperature dependence, with peak zT values of 0.17, 0.18, and 0.14 for Ti₄O₇, Ti₅O₉, and Ti₆O₁₁, respectively, at 1100 K. This variation in thermoelectric performance was mainly attributed to the synergistic effect of the oxygen vacancy concentration and the shear surface density on the carrier concentration and lattice thermal conductivity. Furthermore, the Fermi energy levels and electronic thermal conductivity of the Magnéli phases were calculated using the single parabolic band (SPB) model.