Spin susceptibility of boron carbides: Dissociation of singlet small bipolarons

Using ESR, the spin susceptibilities of a crystal and ceramics of boron carbides have been measured between 4 and 1200 K. A thermally generated contribution to the spin susceptibility, which may be discerned emerging above 100 K, becomes unmistakable above 300 K. The {ital g} values of these spins differ from those of the temperature-independent background spins that dominate low-temperature ESR measurements. Thus, ESR measurements at temperatures well above those of previous works unearth thermally generated spins in boron carbides. We suggest that these thermally induced spins arise from thermal dissociation of boron carbides{close_quote} singlet small bipolarons. The energy with which a small bipolaron is bound with respect to dissociating into two small polarons is estimated to be about 0.2 eV. {copyright} {ital 1996 The American Physical Society.}