Structural and electronic properties of tin clathrate materials

Using an ab initio local density approximation (LDA) method, we have studied the structural properties and the electronic band structures of several tin-based clathrate materials. We find that the pure type-I (Sn₄₆) and type-II (Sn₁₃₆) clathrate frameworks are metastable, expanded-volume phases of Sn and that they are semiconductors. We further find that the Cs encapsulated type-I binary clathrate is metallic if full stoichiometry (Cs₈Sn₄₆) is assumed. However, our calculations suggest that vacancy formation is energetically favored, and that the stable vacancy compound Cs₈Sn₄₄₂ is a narrow gap semiconductor. We have also studied the ternary clathrate compounds Cs₈Ga₈Sn₃₈ and Cs₈Zn₄Sn₄₂, which have been suggested as potential thermoelectrics, and we find that these are semiconductors. The trends in the structural and electronic properties of these Sn-based clathrates are discussed, and our results are compared with experiment where possible.