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 Cs8Sn44h2 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.