Vibrational Contribution to the Seebeck Coefficient of Bipolaronic Carriers in Boron Carbides

Abstract

The Seebeck coefficients (thermoelectric powers) of boron carbides are unusual in that they 1. have large values (> 200 μV/K) despite the presence of high carrier densities (> 1021 cm—3) and 2. have temperature dependencies that are qualitatively different from those characterizing common conductors and semiconductors. These novel features have made boron carbides attractive candidates for use in high-temperature thermoelectric energy conversion. Here the origin of the anomalous Seebeck coefficients of boron carbides is investigated. Attention is focused on how the interactions between electronic carriers and the lattice affect the Seebeck coefficient since prior studies indicate that in boron carbides carriers are singlet bipolarons. It is concluded that an especially significant carrier-induced softening of its surroundings, associated with singlet-bipolaron formation in boron carbides, can account for the surprising thermoelectric properties of boron carbides.