Characterization and analysis of thermoelectric transport in n -type Ba8 Ga16-x Ge30+x

The thermoelectric transport properties of polycrystalline, n -type Ba8 Ga16-x Ge30+x were characterized from 300 to 1000 K. The carrier density was found to vary precisely with the experimental x as expected from simple electron counting. The experimental data are analyzed within the framework of a single parabolic band model, which is found to accurately describe transport for the compositions of interest for thermoelectric application. The lattice thermal conductivity, calculated with a degeneracy adjusted Lorenz number, does not show a trend with composition and a value of ∼1 Wm-1 K-1 is observed at 300 K. A maximum figure of merit zT=0.86 is obtained at 950 K, and the optimal doping level for thermoelectric application is predicted to be ∼2× 1020 cm-3, which corresponds to Ba8 Ga15.75 Ge30.25 by electron counting. An unexpected transition event is observed near 650 K, which results in a significant increase in the heat capacity.