Thermally driven p–n–p multiple heteromorphic transformation facilitating high Seebeck coefficient for Ca0.8La0.2CeNbWO8+δ

Given that different types of carriers tend to gather at the cold end to offset part of the induced voltage, the high Seebeck coefficient obtained by a single type of carrier brings voltage difference to the automatic sensor. Nevertheless, due to the significant charge compensation effect, the effective mass and average energy of carriers can be improved, and a record high Seebeck coefficient (S > 10 mV K−1) is excavated through heteromorphic transformation. Herein, by introducing Ce4+/Ce3+ redox pairs into the p-type doped scheelite CaCeNbWO8 with excellent structural/thermal stability, we can realize various types of p–n–p conversion under thermal driving, and each transformation ushers in a sharp increase in the Seebeck coefficient and conductivity simultaneously. The heteromorphic transition activates solid solution diffusion, promotes transition from variable range hopping transport to nearest neighbor hopping transport, and induces energy-level degeneracy. In the process of p–n transition, the carrier mobility increases sharply and the Seebeck coefficient is ∼5.75 mV K−1 and the conductivity is ∼0.06 S m−1 at 881 K, while metallization occurs during n–p transition accompanying a surge in carrier concentration, resulting in a maximum Seebeck coefficient of ∼17.35 mV K−1 and a conductivity of ∼0.08 S m−1 at 1068 K. The important role of multiple heteromorphic transformation in improving the Seebeck coefficient is revealed, which is expected to be used in low-cost, low pollution, and low-power thermoelectric devices.