First principles study of defect formation in thermoelectric zinc antimonide, β-Zn4Sb3

Understanding the formation of various point defects in the promising thermoelectric material, beta-Zn4Sb3, is crucial for theoretical determination of the origins of its p-type behavior and considerations of potential n-type dopability. While n-type conductivity has been fleetingly observed in Te:ZnSb, there have been no reports, to the best of our knowledge, of stable n-type behavior in beta-Zn4Sb3. To understand the origin of this difficulty, we investigated the formation of intrinsic point defects in beta-Zn4Sb3 density functional theory calculations. We found that a negatively charged zinc vacancy is the dominant defect in beta-Zn4Sb3, as it is also in ZnSb. This explains the unintentional p-type behavior of the material and makes n-doping very difficult since the formation of the defect becomes more favorable at higher Fermi levels, near the conduction band minimum (CBM). We also calculated the formation energy of the cation dopants: Li, Na, B, Al, Ga, In, Tl; of these, only Li and Na are thermodynamically favorable compared to the acceptor Zn vacancy over a range of Fermi levels along the band gap. Further analysis of the band structure shows that Li:Zn4Sb3 has a partially occupied topmost valence band, making this defect an acceptor so that Li: Zn4Sb3 is indeed a p-type thermoelectric material. The introduction of Li, however, creates a more orderly and symmetric configuration, which stabilizes the host structure. Furthermore, Li reduces the concentration of holes and increases the Seebeck coefficient; hence, Li: Zn4Sb3 is more stable and better performing as a thermoelectric material than undoped beta-Zn4Sb3.