Lattice Softening and Band Convergence in GeTe-Based Alloys for High Thermoelectric Performance

GeTe-based alloys have been studied as promising TE materials in the midtemperature range as a lead-free alternate to PbTe due to their nontoxicity. Our previous study on GeTe_1-<i>x</i>I_<i>x</i> revealed that I-doping increases lattice anharmonicity and decreases the structural phase transition temperature, consequently enhancing the thermoelectric performance. Our current work elucidates the synergistic interplay between band convergence and lattice softening, resulting in an enhanced thermoelectric performance for Ge_1-<i>y</i>Sb_<i>y</i>Te_0.9I_0.1 (<i>y</i> = 0.10, 0.12, 0.14, and 0.16). Sb doping in GeTe_0.9I_0.1 serves a double role: first, it leads to lattice softening, thereby reducing lattice thermal conductivity; second, it promotes a band convergence, thus a higher valley degeneracy. The presence of lattice softening is corroborated by an increase in the internal strain ratio observed in X-ray diffraction patterns. Doping also introduces phonon scattering centers, further diminishing lattice thermal conductivity. Additionally, variations in the electronic band structure are indicated by an increase in density of state effective mass and a decrease in carrier mobility with Sb concentration. Besides, Sb doping optimizes the carrier concentration efficiently. Through a two-band modeling and electronic band structure calculations, the valence band convergence due to Sb doping can be confirmed. Specifically, the energy difference between valence bands progressively narrows upon Sb doping in Ge_1-<i>y</i>Sb_<i>y</i>Te_0.9I_0.1 (<i>y</i> = 0, 0.02, 0.05, 0.10, 0.12, 0.14, and 0.16). As a culmination of these effects, we have achieved a significant enhancement in <i>zT</i> for Ge_1-<i>y</i>Sb_<i>y</i>Te_0.9I_0.1 (<i>y</i> = 0.10, 0.12, 0.14, and 0.16) across the entire range of measured temperatures. Notably, the sample with <i>y</i> = 0.12 exhibits the highest <i>zT</i> value of 1.70 at 723 K.