Structure and physical properties of type-I clathrate solid-solution Ba8PtxGe46-x-y□y (□= vacancy)

Starting from binary Ba8Ge433 a series of clathrates was synthesized forming a solid solution Ba8PtxGe46−x−yy, with a solubility limit of 3.5 Pt atoms/f.u. at 800 °C. Throughout the homogeneity region cubic primitive symmetry consistent with the clathrate type I has been confirmed. Phase relations were derived at subsolidus temperatures as well as at 800 °C. Physical properties, such as charge carrier density and transport quantities, were found to be heavily influenced by the Ge/ Pt substitution in the clathrate framework. From electrical resistivity data the series can be classified to be located close to a semiconducting regime with a gap in the electronic density of states of a few thousand K. The gradual change observed is a function of the combined effect of Ge/ Pt substitution and the reduction of vacancies. Thermal conductivity data are modeled in terms of Callaway and von Baeyer’s theory of heat carrying phonons. The efficiency of scattering vacancies is evidenced from the thermal conductivity study. Thermopower indicates that the majority charge carriers are electrons, with a slight variation of the charge carrier density being in the order of n(10^21 cm−3) as a function of the Pt content. Additionally, S(T) basically defines the figure of merit Z·T, which represents the thermoelectric performance of a certain material. Using the compound with the highest Seebeck coefficient, i.e., x=2.7 reveals ZT about 0.15 at 600 K.