Thermoelectric properties of rare earth-ruthenium-germanium compounds

We report our studies of the thermoelectric properties of a class of compounds of composition R3Ru4Ge13 (R=Y, Dy, Ho, and Lu). Magnetization measurements show that the R ion is in the trivalent state in each of these compounds. Each of these compounds displays a semiconductorlike rise in electrical resistivity with decreasing temperature. The magnitude of the resistivity is much larger than typical metals and is similar to that of a heavily doped semiconductor or semimetal. The Seebeck coefficient is positive throughout the temperature range of 2-800 K with room temperature values of approximately 40 mu V K-1. The lattice thermal conductivity is remarkably low and exhibits a very flat temperature dependence. This is consistent with the observed internal disorder associated with the cagelike structure of these compounds. Cobalt substitution for ruthenium enhances the semiconductor character but does not improve the thermoelectric properties. (C) 2007 American Institute of Physics.