Thermopower and electrical conductivity of Ca3-xYxMn2Ge3O12 garnet

The conductivity and thermopower of several electronically conducting, polycrystalline Ca3-xYxMn2Ge3O12 samples in the composition range 0=x=2 have been measured in the temperature range 30-1000 °C. The conductivities are thermally activated while the thermopower is temperature independent. These results are shown to be consistent with adiabatic hopping of small polarons localized on the manganese sublattice. The number of available transport sites is reduced by defect interactions. The mobilities are thermally activated with Ea~0.62 eV for 0=x=1.75 and Ea=0.79 eV for x=2. The calculated optical-phonon frequencies agree well with the recorded infrared lattice spectra.