Effect of defect interactions with interstitial Ag in the lattice of Bi_xSb_2-xTe₃alloys and their thermoelectric properties

In this study, the effect of Ag defects on the lattice and thermoelectric properties of Ag-doped BixSb2-xTe3 (BST) alloys are systematically investigated. Using density-functional calculations, we reveal that Ag impurity defects occupying interstitial sites in the BST affect the formation of intrinsic defects in the host materials. Ag interstitial defects, which are responsible for increasing the lattice parameter of the c axis, lower the formation energies of BiTe and SbTe acceptor defects more effectively in the Bi-rich BST than in the Bi-poor BST. In addition, these Ag interstitials induce the formation of AgBi and AgSb in BixSb2-xTe3. Therefore, Ag-induced defect interactions can be used to finely optimize the defects and doping density by varying the Bi content. Furthermore, the phonon thermal conductivity is reduced with the formation of nanoscale twin structures with various stacking faults. Finally, a high figure of merit of 1.4 at 423 K is achieved for p-type BST alloys.