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

Skutterudite CoSb 3 based thermoelectric devices have high potential for engineering applications because both n- and p-type doped CoSb 3 demonstrate excellent thermoelectric performance. A crucial point concerning the application of CoSb 3 is to understand and control its defect chemistry. To reveal the native conductivity behavior of nonstoichiometric CoSb 3, we investigated the intrinsic point defects in CoSb 3 using density functional theory. We found CoSb 3 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 CoSb 3 . The unexpected acceptor nature of the Co i or Co i-p defects is explained by the breakage of multiple Sb 4 -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 CoSb 3 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 .