p-Type Co Interstitial Defects in Thermoelectric Skutterudite CoSb₃ Due to the Breakage of Sb₄-Rings

Skutterudite CoSb3 based thermoelectric devices have high potential for engineering applications because both n- and p-type doped CoSb3 demonstrate excellent thermoelectric performance. A crucial point concerning the application of CoSb3 is to understand and control its defect chemistry. To reveal the native conductivity behavior of nonstoichiometric CoSb3, we investigated the intrinsic point defects in CoSb3 using density functional theory. We found CoSb3 is p-type in either Co or Sb rich regions of phase stability. Interstitial Co (Coi) and interstitial Co-pair (Coi-p) are the dominant point defects in the Co rich region. However, Coi-p will be difficult to form because the formation temperature of Coi-p is much lower than the synthesis temperature of CoSb3. The unexpected acceptor nature of the Coi or Coi-p defects is explained by the breakage of multiple Sb4-rings. Co vacancy (Cov) is found to be the p-type defect in the Sb rich region. Furthermore, the solubility of excess Co in CoSb3 is expected to be larger than that of Sb because of the lower formation energy and higher carrier concentration of Coi compared with those of Cov.