Thermodynamic analysis of the filling fraction limits for impurities in CoSb3 based on ab initio calculations

Abstract The thermodynamic stabilities of alkaline earth (Ca, Sr and Ba), and rare earth (La, Ce and Yb), filled CoSb 3 skutterudites have been studied using a plane-wave density functional method. By combining the formation energy of inserting an impurity into the intrinsic void of CoSb 3 and that of secondary phases I Sb 2 and CoSb 2 , it is found that the filling fraction limit (FFL) or the maximum filling fraction of an impurity I corresponds to the minimum formation energy for a mixed chemical reaction route that results in the formation of filled skutterudite I y Co 4 Sb 12 at the maximum filling as well as the formation of secondary phases. Theoretically estimated FFLs of various impurities in the voids of CoSb 3 are in good agreement with the reported experimental data. A schematic phase diagram for filled CoSb 3 is given. Discussion on the effect of the ionic radius of a filler and the content of Sb on FFL is presented.