An exploration of noble metal substitution in germanium based clathrates

The thermopower (S) is sensitive to changes in the density of states (g(epsiv)) near the Fermi level (epsiv_F). Strongly correlated electron systems with transitions metals and/or rare earth elements exhibit sharp density of states features near the Fermi level due to the d and f states. This leads to the large thermopowers often found in these compounds. A wide variety of germanium based clathrates containing transition metals form (Ba₈TM_-xGe_46-x where TM = Ni, Pd, Pt, Cu, Ag and Au are known to exist). Enhancement of the thermopower while retaining the low thermal conductivity and relatively high electrical conductivity characteristic of the clathrates will improve thermoelectric performance of these compounds. As part of an ongoing investigation of transition metal substituted clathrates, in this paper we present a characterization of the structural and electronic transport properties of two p-type silver and gold substituted samples. Recently the thermoelectric properties of several Ba₈Au_xGe_46-x samples were presented in a proceedings paper, but to the best of our knowledge this is the first time thermoelectric properties are presented for Ba₈Ag₆Ge₄₀.