Enhanced thermoelectric performance in GeTe-Sb2Te3 pseudo-binary via lattice symmetry regulation and microstructure stabilization

Abstract

Pseudo-binary GeTe-rich Sb2Te3(GeTe)n materials recently exhibited promising thermoelectric performance at intermediate temperatures (500–800 K), largely due to the intrinsically low lattice thermal conductivity coming from the discrete van der Waals gaps dispersed in a rhombohedral matrix. In this work, by alloying Ge with Pb and adjusting the molar ratio of GeTe/Sb2Te3 in the binary, we successfully modulated the crystal structure from rhombohedral Sb2Te3(GeTe)17 to pseudo-cubic (Sb2Te3)0.5(Ge0.91Pb0.09Te)17.5 at room temperature, thus achieved higher electronic band degeneracy and electrical performance. High-resolution scanning transmission electron microscope (STEM) characterizations revealed the existence of high-density discrete van der Waals gaps (length ∼ 10–40 nm) along {111} equivalent planes in GeTe matrix; surprisingly, these planar defects appear quite stable in following annealing processes at 873 K unlike what literatures reported. Further elemental mapping suggests that the enrichment of Pb element around van der Waals gaps are possibly responsible to the formation and stabilization of these planar defects. Eventually, a figure of merit ZTmax ∼2.4 at 773 K and average ZTavg ∼1.5 at 323–773 K were simultaneously realized in the (Sb2Te3)0.5(Ge0.91Pb0.09Te)17.5 sample after 4 days annealing at 873 K.