Mixed-stack Charge-transfer Crystals Research Articles

Intrinsic negative-resistance effect in mixed-stack charge-transfer crystals.

A stable and reproducible negative-resistance effect has been observed in various mixed-stack organic charge-transfer crystals having ionicities close to the neutral-ionic phase boundary. Systematic studies on several kinds of single crystals indicate that this effect is an intrinsic property, arising from the dynamics of the charge-carrying defects with fluctuating molecular valence in the one-dimensional (1D) molecular stacks. A microscopic mechanism of the negative resistance is proposed taking account of the three-dimensional electrostatic interaction between 1D stacks.

Raman studies of the pressure driven neutral to ionic transitions in tetrathiafulvalene-haloquinone mixed stack charge transfer crystals

Pressure-induced neutral to ionic phase transitions in mixed stack charge transfer crystals have been studied by Raman spectroscopy. In the case of tetrathiafulvalene-chloranil, an accurate determination of the ionicity variation with pressure has yielded a precise characterization of the phase transition, which occurs discontinuously at about 1.1 GPa. The possible presence of ionic dimers embedded in the neutral chains as precursors of the phase change is discussed. Preliminary measurements on the rather similar compound tetrathiafulvalene-2,5dichloroparabenzoquinone suggest the possibility of a continuous neutral to ionic transition.

TMPD–CA revisited: Ionicity, stack dimerization, and phase transition of a key mixed stack charge transfer crystal

Raman and single crystal polarized reflectance spectra (from 400 to 10 000 cm−1) of the tetramethylparaphenylenediamine–chloranil (TMPD–CA) charge transfer complex are reported. A careful interpretation of the data leads one to estimate for TMPD–CA a degree of ionicity of 0.64 at room temperature, indicating a quasi-ionic ground state. Moreover, we show that the mixed stack formed by TMPD and CA is dimerized at all temperatures. The latter finding is substantiated by an a priori calculation of the infrared conductivity spectrum polarized along the stack axis. The calculation is based on a model taking into account the coupling of the molecular vibrations with the charge transfer excitations through the modulation of both on-site energies and charge transfer integrals; moreover the intrinsic intensity of the infrared active modes is introduced semiempirically. The spectral simulation finds a further application in interpreting the subtle spectral changes observed in correspondence with the TMPD–CA 250 K phase transition: it is suggested that the phase transition is a disorder–order, orientational one. An extensive comparison with the results previously obtained by other experimental techniques allows one to solve early contradictions and definitely clear up the nature of the ground state of this key charge transfer crystal.

Zero-temperature phase diagram of mixed-stack charge-transfer crystals.

The zero-temperature phase diagram of mixed-stack charge-transfer crystals is investigated through a diagrammatic valence-bond technique. The direct solution of the single-chain Hubbard Hamiltonian inclusive of intersite Coulomb interactions constitutes the basis for a perturbative, adiabatic treatment of the electron--lattice-phonon and electron--molecular-vibration couplings. It is shown that intersite Coulomb interactions and electron--molecular-vibration coupling cooperate in favoring uneven distribution of the electronic charge on the sites, possibly giving rise to a discontinuous, first-order neutral-ionic (N-I) phase transition. On the other hand, the electron--lattice-phonon coupling favors an uneven distribution of the charge between the sites, yielding a Peierls-type dimerization instability. However, when all the interactions are turned on, a rather subtle feedback interplay is found, the dimerization instability range being widened by the Coulomb interactions and the electron--molecular-vibration coupling. The resulting four-dimensional phase diagram is able to account for the experimentally observed ground states and phase transitions of mixed-stack charge-transfer crystals. In particular, it is found that the boundary between N regular stack phases and I dimerized ones is, in general, very complex, as several minima in the potential-energy curve are available to the system. Consequently, the corresponding phase transition is very sensitive to the experimental conditions, a situation we believe is encountered in the famous N-I transition of tetrathiafulvalene-chloranil. In addition, preliminary calculations on dimerized stack systems show that whereas ionic regular chains are intrinsically unstable towards dimerization, the corresponding energy gain decreases as the ionicity increases. Therefore at finite temperatures ionic regular stacks may be observed, their transition temperature to the dimerized phase being expected to be lower with increasing ionicity.

Regular-dimerized stack vs neutral-ionic instability in mixed stack CT crystals

Valence bond calculations are used to investigate the interplay between regular-dimerized stack and neutral-ionic interfaces in mixed stack charge transfer (CT) crystals. Direct solution of the hamiltonian comprehensive of intersite Coulomb interactions, and a perturbative approach for the electron-phonon and electron-molecular vibrations couplings allow us to assess the role of these interactions. The resulting phase diagram nicely accounts for the experimental findings.

Discovery of vibronic effects in the Raman spectra of mixed-stack charge-transfer crystals

By studying the tetrathiafulvalene-chloranil (TTF-CA) complex the different of electron---molecular-vibration ($e$-mv) interactions in the Raman and infrared spectra of mixed-and segregated-stack one-dimensional compounds are compared and interpreted. In mixed-stack crystals, besides the well-known infrared effects, the vibronic coupling causes a shift of the Raman frequencies. From this previously unobserved effect the $e$-mv coupling constants of TTF and CA are evaluated.