Intrinsic paramagnetic defects have been detected by ESR measurements in tetrathiafulvalene -$p$- chloranil single crystals, associated with the neutral-to-ionic phase transition at $T_c=84\mathrm{}$ K. Two kinds of paramagnetic species are identified with delocalized spins propagating on either donor or acceptor sites. Total spin density and electric conductivity were found to increase discontinuously as temperature is lowered across $T_c$. These features are attributed to the formation of mobile solitonlike (and antisolitonlike) paramagnetic defects induced in the dimerized lattice at the phase transition.

• Received 23 January 1984

DOI:https://doi.org/10.1103/PhysRevLett.53.842