Synthesis, Transport and Magnetic Properties of Ba-Co-Ge Clathrates

Abstract

Ba-Co-Ge intermetallic clathrates were synthesized in both type-I and chiral-type crystal structures, to investigate thermoelectric and physical properties. Seebeck coe cients, thermal conductivities, electrical resistivities, and Hall coe cients were measured, as well as specific heat and magnetic susceptibility. Type-I Ba 8 Co x Ge 46 y was formed with a large number of spontaneous vacancies, similar to a Zintl condition, but without the vacancy ordered superstructure of Ba 8 Ge 43. 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 e↵ective carrier density, a behavior consistent with other materials close to Ba 8 Ge 43. In the case of chiral samples, Co substitution strongly suppresses 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 e↵ect of these transitions compared to the case of unsubstituted Ba 6 Ge 25 .