Systematic Study of the Multiple-Element Filling in Caged Skutterudite CoSb3

Impurity filling fraction limits (FFLs) in nearly 20 dual-element-filled CoSb3 are investigated via ab initio density functional calculations. The two types of filling impurities are selected from rare-earths (La, Ce, Eu, Yb), alkaline-earths (Ba, Sr), and alkaline metals (Na, K) with different chemical nature. Our results show that alkaline-earth and alkaline metal atoms can be jointly filled into the crystal voids, and the total filling fraction limits follow a linear correlation rule, that is, the filling fraction of one element decreases linearly whereas the other element's increases. Cofilling of alkaline-earth (or alkaline metal) with rare-earth shows one filler dominant, usually alkaline-earth (or alkaline metal), or the one with relatively high filling fraction limit. Rare-earth impurities as cofilling atoms have relatively lower filling fractions than alkaline-earth (or alkaline metal) atoms. Eu has a relatively high FEL comparable to the FFLs of alkaline-earths, and the total FEL for the Na Eu cofilled CoSb3 may be higher than that of the Na-single-filled CoSb3 (65%, the highest FFL). A model to estimate the FFLs for the filling with more than two types of elements in filled skutterudites is also discussed.