Localized crystal imperfections coupled with phase diagram engineering yield high-performance rhombohedral GeTe thermoelectrics

Abstract

Cubic β-GeTe alloys are apt to hoist the conversion efficiency for mid-temperature thermoelectric (TE) generators with their remarkably high peak figure-of-merit (zT) values at T > 700 K. Nevertheless, the TE device that consolidated by β-GeTe alloys have existed concerns as the rhombohedral α-GeTe to β-GeTe phase-transition leverage over the workability and thermal stability. Therefore, research efforts are redirected towards the high-performance α-GeTe, which stabilizes at a rhombohedral lattice below 700 K. By incorporating the maximal Bi solubility, the α-GeTe shows severe lattice distortion without forming the undesired impurities that affect the transport properties and deteriorate the thermal stability. The α-GeTe with high-dose Bi fulfills the counterbalance between low thermal conductivity κ and elevated power factor PF = S2ρ−1. Herein, the Bi0.1Ge0.9Te crystal attains the peak zT of 1.5 at 625 K and 1.9 at 713 K for its rhombohedral and cubic state, respectively. The Bi0.1Ge0.9Te features hierarchical twinning accompanied with dense stacking faults which explains its ultra-low lattice thermal conductivity κL in the temperature range of 300 K–700 K. The trade-off between incompatible low-κ and high-PF could be optimized via the phase diagram and defect engineering, which synergistically open a new category for TE performance advancement.