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 (Co_i) and interstitial Co-pair (Co_i-p) are the dominant point defects in the Co rich region. However, Co_i-p will be difficult to form because the formation temperature of Co_i-p is much lower than the synthesis temperature of CoSb₃. The unexpected acceptor nature of the Co_i or Co_i-p defects is explained by the breakage of multiple Sb₄-rings. Co vacancy (Co_v) 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 Co_i compared with those of Co_v.