Structure and Physical Properties of Clathrate I Systems Ba8PdxSi46-x and Ba8PtxSi46-x

Phase relations at 900 degrees C, crystal chemistry, electrical and thermal transport measurements, and heat capacity data are reported for the clathrate systems: Ba-8{Pd, Pt}(x)Si46-x. For both systems a ternary clathrate phase was identified with a small homogeneity region extending at 900 degrees C for 2.5 <= xPd <= 4.1 and 2.8 <= xPt <= 4.9, respectively. Structural investigations of these clathrate phases at 300, 200 and 100 K define cubic primitive symmetry with the space group type Pm (3) over barn consistent with clathrate type I. Studies of electronic and thermal transport groups both series of compounds close-by a semiconducting state. An sizeable charge carrier concentration in these systems, however, prevents sufficiently large thermopower values. In comparison to isostructural type I clathrates based on Ge, thermal conductivity - dominated by the lattice contribution - is larger due to the smaller mass of Si.