Discordant Gd and Electronic Band Flattening Synergistically Induce High Thermoelectric Performance in n-type PbTe

p-type PbTe is the most sought-after material in thermoelectrics due to its ultrahigh thermoelectric figure of merit (zT), but the performance of n-type PbTe lags behind due to the simplicity of its conduction band compared to those of rich valence bands. Here, we have synergistically enhanced the Seebeck coefficient and lowered the lattice thermal conductivity of n-type PbTe by Gd doping, which resulted in high thermoelectric performance. Gd doping in PbTe enhances the electron effective mass via flattening of the conduction band, which significantly improves the Seebeck coefficient. Gd is found to remain in an off-centered position inside the rock-salt PbTe lattice, which increased the lattice anharmonicity. Density functional perturbation theory (DFPT) calculations indicate that Gd induces a low energy nearly flat optical phonon mode that drastically increases the phonon scattering rate and lowers lattice thermal conductivity to 0.78 W/(m K). As a result, we achieved a high zT of 1.65 at 678 K for Pb1–xGdxTe1–yIy (x = 0.33%, y = 1%).