High Thermoelectric Performance of p-Type PbTe Enabled by the Synergy of Resonance Scattering and Lattice Softening

In this work, we show the simultaneous enhancement of electrical transport and reduction of phonon propagation in p-type PbTe codoped with Tl and Na. The effective use of advanced electronic structure engineering improves the thermoelectric power factor S²σ over the temperature range from 300 to 825 K. A rise in the Seebeck coefficient S was obtained due to the enhanced effective mass m*, coming from the Tl resonance state in PbTe. Due to the presence of additional carriers brought by Na codoping, electrical conductivity became significantly improved. Furthermore, Tl and Na impurities induced crystal lattice softening, remarkably reducing lattice thermal conductivity, which was confirmed by a measured low speed of sound v_m and high internal strain Cε_XRD. Eventually, the combination of both the attuned electronic structure and the lattice softening effects led to a very high ZT value of up to ∼2.1 for the Pb_1-TlNaTe samples. The estimated energy conversion efficiency shows the extraordinary value of 15.4% (T_c = 300 K, T_h = 825 K), due to the significantly improved average thermoelectric figure of merit ZT_ave = 1.05. This work demonstrates that the combination of impurity resonance scattering and crystal lattice softening can be a breakthrough concept for advancing thermoelectrics.