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+^ρ per site. For ^ρ=0 spontaneous bond dimerization and soliton excitations are possible if α<Δ₀, where α is the intrinsic gap and Δ₀ is the spontaneous dimerization gap when α=0. Recently observed neutral-to-ionic transitions in mixedstack organic compounds are reinterpreted as the onset of such bond dimerization. For ^ρ≠0 spontaneous order and a soliton lattice are found even if α>~Δ₀. The electronic gap then has a maximum at a finite ^ρ.