Regular-dimerized stack and neutral-ionic interfaces in mixed-stack organic charge-transfer crystals

Using diagrammatic valence-bond calculations, we have investigated the interplay between neutral-ionic (N-I) and regular-dimerized stack interfaces in mixed-stack organic charge-transfer (CT) crystals. The interactions relevant to the above two interfaces, that is, the intersite Coulomb and the electron-lattice phonon couplings, are introduced via a mean-field approach and a perturbative Herzberg-Teller expansion, respectively. The k=0 results for finite chains (up to scrN=12 sites) and rings (up to scrN=14 sites) are extrapolated to scrN{→}{∞}, obtaining an appropriate description of the electronic structure of a mixed, regular chain. The calculations distinguish the N and I phases as characterized, respectively, by nondegenerate and degenerate singlet ground states, the crossing point being found at a degree of ionicity ({ρ}) of about 0.63. Also the singlet-triplet gap vanishes in the I phase. The explicit consideration of the intersite Coulomb interactions changes the N-I interface from a continuous to a discontinuous one (first-order phase transition), whereas the electron-lattice phonon coupling makes the singlet degenerate (or quasidegenerate) ground state subject to the Peierls instability. The results are summarized in terms of a three-dimensional phase diagram relating {ρ}, the lattice-distortion energy, and the stabilization energy of the ionic stack due to intersite Coulomb interactions. This phase diagram is shown to nicely account for several experimental observations relevant to mixed-stack CT crystals and to their phase transitions.