SPIN SUSCEPTIBILITY OF BORON CARBIDES - DISSOCIATION OF SINGLET SMALLBIPOLARONS

Using ESR, the spin susceptibilities of a crystal and ceramics of boro n carbides have been measured between 4 and 1200 K. A thermally genera ted contribution to the spin susceptibility, which may be discerned em erging above 100 K, becomes unmistakable above 300 K. The g values of these spins differ from those of the temperature-independent backgroun d spins that dominate low-temperature ESR measurements. Thus, ESR meas urements at temperatures well above those of previous works unearth th ermally generated spins in boron carbides. We suggest that these therm ally induced spins arise from thermal dissociation of boron carbides' singlet small bipolarons. The energy with which a small bipolaron is b ound with respect to dissociating into two small polarons is estimated to be about 0.2 eV.