Identifying polymorphic lattice positioning of copper and the effects on the thermoelectric performance of δ-LAST

Abstract

N-type AgPbmSbTem+2 (abbreviated as LAST-m) has attracted much attention as an important branch of PbTe-based thermoelectric materials, mainly due to its complex microstructure and ultralow thermal conductivity. However, LAST-m met much difficulty in repeatability and reproducibility of thermoelectric performance largely due to the meta-stable feature of the component AgSbTe2. In this work, we replaced stoichiometric AgSbTe2 with non-stoichiometric Ag0.352Sb0.549Te, and alloyed it with PbTe to get thermodynamically stable and single-phase δ-LAST solid solutions. Further addition of extra Cu with excess dose results in the polymorphic lattice positioning of Cu atoms in the matrix of δ-LAST as revealed by Cs-corrected scanning transmission electron microscope (STEM), i.e., single Cu interstitial, Cu interstitial monolayer, Cu interstitial bilayer, Cu interstitial super-lattice, Cu interstitial intermediate compound and Cu2-xTe. It was argued that the polymorphic positioning of Cu atoms not only dynamically optimize the electron concentration at climbing temperatures but also effectively scatter heat-carrying phonons in a panoscopic approach, leading to an outstanding thermoelectric performance competitive with state-of-art LAST-m and other n-type PbTe-based thermoelectrics.