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

The thermodynamic stabilities of alkaline earth (Ca, Sr and Ba), and rare earth (La, Ce and Yb), filled CoSb3 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 CoSb3 and that of secondary phases ISb2 and CoSb2, 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 IyCo4Sb12 at the maximum filling as well as the formation of secondary phases. Theoretically estimated FFLs of various impurities in the voids of CoSb3 are in good agreement with the reported experimental data. A schematic phase diagram for filled CoSb3 is given. Discussion on the effect of the ionic radius of a filler and the content of Sb on FFL is presented.