Bipolaron hopping conduction in boron carbides

The electrical conductivities of boron carbides, B12+xC3-x With 0.1 < x < 1.7, between 300 and 1200 K suggest the hopping of a nearly temperature-independent density of small (bi)polarons. The activation energies of the mobilities are low, approximate to 0.16 eV, and are nearly independent of the composition. At lower temperatures, conductivities have non-Arrhenius temperature dependencies and strong sensitivity to carbon concentration. Percolative aspects of low-temperature hopping are evident in this sensitivity to composition. Boron carbides' Seebeck coefficients are anomalous in that 1. they are much larger than expected from boron carbides' large carrier densities and 2. they depend only weakly on the carrier density. Carrier-induced softening of local vibrations gives contributions to the Seebeck coefficient that mirror the magnitudes and temperature dependencies found in boron carbides.