Engineering Thermoelectric Performance of α-GeTe by Ferroelectric Distortion

The rhombohedral α-GeTe can be approximated as a slightly distorted rock-salt structure along its [1 1 1] direction and possesses superb thermoelectric performance. However, the role of such a ferroelectric-like structural distortion on its transport properties remains unclear. Herein, we performed a systematic study on the crystal structure and electronic band structure evolutions of Ge_1-xSn_xTe alloys where the degree of ferroelectric distortion is continuously tuned. It is revealed that the band gap is maximized while multiple valence bands are converged at x = 0.6, where the ferroelectric distortion is the least but still works. Once undistorted, the band gap is considerably reduced, and the valence bands are largely separated again. Moreover, near the ferro-to-paraelectric phase transition Curie temperature, the lattice thermal conductivity reaches its minima because of significant lattice softening enabled by ferroelectric instability. We predict a peak ZT value of 2.6 at 673 K in α-GeTe by use of proper dopants which are powerful in suppressing the excess hole concentrations but meanwhile exert little influence on the ferroelectric distortion.