Effect of Substitutional Impurities on the Neutral-to-Ionic Phase Transition in TTF-p-Chloranil Crystal

Abstract

In order to investigate the effect of doping on the neutral-to-ionic phase transition, reflection spectra due to the intramolecular excitations have been measured on TTF-p-chloranil single crystals doped with the substitutional weak acceptor, trichloro-p-benzoquinone, at various temperatures and dopant concentrations. The thermal shifts of the spectra are considerably blurred by doping in contrast with a sharp spectral change at T_c=84 K in pure crystal. It is concluded that the phase transition in doped crystals occurs inhomogeneously due to a binary phase separation into quasi-ionic and quasi-neutral regions even below T_c. On the basis of these results, a microscopic model is presented for the effect of impurities on the neutral-to-ionic phase transition in TTF-chloranil crystal.