Diatomic polymers, mixed-stack compounds, and the soliton lattice

The continuum electron-phonon system is studied for (AB)x-type polymers with an average charge of 1+ per site. For =0 spontaneous bond dimerization and soliton excitations are possible if 0, where is the intrinsic gap and 0 is the spontaneous dimerization gap when =0. Recently observed neutral-to-ionic transitions in mixedstack organic compounds are reinterpreted as the onset of such bond dimerization. For 0 spontaneous order and a soliton lattice are found even if 0. The electronic gap then has a maximum at a finite.