Synthesis and properties of barium-germanium clathrates containing transition metals

Abstract

Intermetallic clathrates are known and intensively investigated as promising candidates for thermoelectric applications. On the other hand, several clathrates have attracted attention due to their superconductivity. This thesis focuses on the search for new type-I clathrates and their characterization. For the first time, single phase poly-crystals of Ba8(E9)xGe46-x-y(E9 = Rh, Ir, Co) and bulk single crystals of Ba8Ni3.5Ge42.1 were synthesized. The latter were grown by the Bridgman method and were up to 1 cm in diameter and 1.5 cm in length. All samples were analyzed by XRD, EDX, WDX, and metallography. For the system Ba-Co-Ge and the Ba8Ni3.5Ge42.1 single crystals the potential for thermoelectric applications was investigated by detailed electrical resistivity, Hall effect, thermopower, and thermal conductivity measurements. Trends are discussed and further routes to optimize the thermoelectric figure of merit are suggested.<br />The systems Ba-Rh-Ge and Ba-Ir-Ge show superconductivity with transition temperatures of approximately 7 K and 6 K, respectively. The zero field cooling and field cooling magnetic susceptibility measurements confirm the bulk nature of the superconductivity. The dependence of the critical temperature and the lower and upper critical field on the Rh and Ir content will be discussed in detail based on results obtained from electrical resistivity, specific heat and magnetization measurements.