Filling fraction limits for rare-earth atoms in CoSb3:: An ab initio approach

Filling fraction limits (FFLs) of rare-earth (RE) impurities in the lattice voids of CoSb3 are studied by ab initio density functional calculations. Our calculations reveal that the FFL decreases rapidly from La to Sm, and becomes zero from Gd to Lu, with the exception of Eu and Yb which exhibit relatively high FFLs (0.44 for Eu and 0.30 for Yb). The zero FFL for the heavy RE atoms is attributed to the fact that the formation energies of the corresponding secondary phases are lower than those for filling an isolated impurity into the voids of CoSb3. The reasons for the relatively high FFLs of Yb and Eu are also discussed.