Effects of morphology on the electronic and transport properties of Sn-based clathrates

摘要

Density-functional calculations are used to study the electronic structure and transport properties of the type-I clathrates, K_8Sn_(46), and K_8Sn_(44)square_2, (square is a missing Sn atom), and the type-III clathrate K_8Sn_(25). We show K_8Sn_(44)square_2, to be more stable than the defect-free K_8sN_(46), with K_8Sn_(46) being metallic and K_8Sn_(44)square_2; semimetallic. K_8Sn_(25) is a zintl-phase semiconductor with a band gap of 0.5 eV. It has flatter bands than the type-I clathrates and can be expected to exhibit a smaller electrical conductivity but much larger Seebeck coefficient than the almost zintl K_8Sn_(44)square_2. The figure of merit of the type-III clathrate is an order of magnitude larger than its type-I counterparts. This would make a suitably doped K_8Sn_(25) at least as good a thermoelectric as the best Ge-based type-I clathrates, and potentially better, depending on the momentum relaxation time for carriers in this material.