Synthesis, transport and magnetic properties of Ba-Co-Ge clathrates
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Ba-Co-Ge intermetallic clathrates were synthesized in both type-I and chiral-type crystal structures to investigate thermoelectric and physical properties. Seebeck coefficients, thermal conductivities, electrical resistivities, and Hall coefficients were measured, as well as specific heat and magnetic susceptibility. Type-I Ba8Co x Ge46−y was formed with a large number of spontaneous vacancies, similar to a Zintl condition, but without the vacancy ordered superstructure of Ba8Ge43. However, the vacancies for this composition do not moderate the carrier density as expected from Zintl electron balancing. Instead, the physical properties point to a complex Fermi surface property with a large effective carrier density, a behavior consistent with other materials close to Ba8Ge43. In the case of chiral samples, Co substitution strongly suppresses the temperature of only the lower of the two structural transformations. Susceptibility and specific heat measurements, coupled with the measured transport properties, demonstrate that the electron densities of states near the Fermi energy change very little in the transformations, a significant reduction in the effect of these transitions compared to the case of unsubstituted Ba8Ge25.