Localized double phonon scattering and DOS induced thermoelectric enhancement of degenerate nonstoichiometric Li1-xNbO2 compounds
We report the synthesis and thermoelectric properties of a new p-type oxide thermoelectric material (Li1-xNbO2, with x ¼ 0–0.6), in which Li-vacancies play a significant role in the enhancement of the thermoelectric performance. The electrical conductivity drastically increases due to Li-vacancies, resulting in an increase in the hole carrier concentration. A remarkable enhancement in the power factor is observed, which is comparable to well-known oxide thermoelectrics. The carrier concentration was not significantly dependent on the temperature, while the Hall mobility shows negative temperaturedependence. The Seebeck coefficient is linearly proportional to temperature, and the density of the state effective mass ðm* dÞ was estimated by using the Pisarenko relation. The thermal conductivity was substantially reduced by Li-vacancies and Li-vacancy induced localized double phonon scattering. Density functional theory (DFT) calculations reveal that the enhancement of the thermoelectric properties is mainly due to the increase in the electronic density of states (DOS) at the Fermi energy, which increases with hole concentration. All of the samples after high-temperature measurements are highly stable, which suggests that the well-synthesized nonstoichiometric Li1xNbO2 could be a new promising candidate material for high temperature thermoelectric applications.