Large Enhancement of Boron Carbides' Seebeck Coefficients through Vibrational Softening

We report measurements of the Seebeck coefficients of boron carbides having compositions ${\mathrm{B}}_{12+x}{\mathrm{C}}_{3-x}$, $0.15<x<\mathrm{}1.7\mathrm{}$, from 6 to 900 K. These Seebeck coefficients are large, $\approx 200\mu \mathrm{V}/\mathrm{K}$ at 300 K, despite boron carbides having high carrier densities, $\approx {10}^{21}{\mathrm{cm}}^{-3\mathrm{}}$. The Seebeck coefficients also have unusual temperature dependencies and depend only weakly on carrier density. These exceptional properties can arise from boron carbides' bipolaronic holes substantially softening the vibrational modes with which they interact.