TTF-CA

Neutral-Ionic Transition in TTF-CA

While in the previous example of TMTTF and TMTSF salts the crystals consist of separate cation and anion chains between which the electron transfer occurs, mixed-stack organic charge-transfer compounds have only one type of chain composed of alternating p-electron donor and acceptor molecules (... ADADAD...).

TTF-CA1

The TTF and chloranil QCl4 are planar molecules. In the mixed-stack compound TTF-CA the two distinct molecules alternate.

TTF-CA3

 

Naturally grown single crystal of TTF-CA in the neutral phase (green phase) synthesized according to the plate sublimation technique.

These materials are either neutral or ionic, but under the influence of pressure or temperature certain neutral compounds become ionic. There is a competition between the energy required for the formation of a D+A- pair and the Madelung energy. Neutral-ionic (NI) phase transitions are collective, one-dimensional charge-transfer phenomena occurring in mixed-stack charge-transfer crystals, and they are associated to many intriguing phenomena, as the dramatic increase in conductivity and dielectric constant at the transition.
In the simplest case, the charge per molecule changes from completely neutral ρ=0 to fully ionized ρ=1. Ideally this redistribution of charge is decoupled from the lattice, and therefore should not change the inter-molecular spacing. In most real cases, however, the NI transition is characterized by the complex interplay between the average ionicity ρ on the molecular sites and the stack dimerization δ. The ionicity may act as an order parameter only in the case of discontinuous, first order phase transitions. While the inter-site Coulomb interaction V favors a discontinuous jump of ionicity, the intra-chain charge-transfer integral t mixes the fully neutral and fully ionic quantum states and favors continuous changes in ρ. The coupling of t to lattice phonons induces the dimerization of the stack, basically a Peierls-like transition to a ferroelectric state, which is a second order phase transition. Intramolecular (Holstein) phonons, on the other hand, modulate the on-site energy U and favor a discontinuous jump in ρ.

The temperature induced NI transition of tetrathiafulvalene-tetrachloro-p-benzoquinone (TTF-CA) at TNI = 81 K is the prime example of a first-order transition with a discontinuous jump in ρ. This can be seen be an abrupt change in the optical properties; below the NI transition the coupled bands shift to higher frequencies. In terms of a modified, one-dimensional Hubbard model the NI transition can be viewed as a transition from a band insulator to a Mott insulator due to the competition between the energy difference between donor and acceptor sites, and the on-site Coulomb repulsion U. Peierls and Holstein phonons are both coupled to charge transfer electrons, albeit before the NI transition the former are only infrared active, and the latter only Raman active. This makes polarized Raman and reflection measurements a suitable tool to explore the NI transition.

The optical experiments identify practically all the totally symmetric modes of both neutral and ionic phases of TTF-CA. The vibronic bands present inthe infrared spectra for T > TNI are due to sum and difference combinations involving the lattice mode, which gives rise tothe Peierls distortion at the transition. Three lattice modes which couple to electrons and become stronger as the transition is approached; they behave as soft modes of the ferroelectric transition at TNI= 81 K. The lowest mode softens most and is seen strongly overdamped around 20 cm-1. The temperature evolution of this Peierls mode, which shows a clear softening before the first-order transition to the ionic ferroelectric state takes place. In the ordered phase a clear identification and theoretical modelling of the Goldstone mode is still an open problem because the system has several degrees of freedom coupled to each other.The cooperative charge transfer among the constructive molecules of TTF-CA can also be induced by irradiation of a short laser pulse. A photoinduced local charge-transfer excitation triggers the phase change and cause the transition in both directions. When Cl is replaced by Br in the tetrachloro-p-benzoquinones the lattice is expanded, (like a negative pressure) and the ionic phase vanishes completely. Hydrostatic pressure or Br-Cl substitution is utilized as a control parameter to more or less continuously tune the NI transition at T → 0.
In the simplest case, the charge per molecule changes from completely neutral ρ=0 to fully ionized ρ=1. Ideally this redistribution of charge is decoupled from the lattice, and therefore should not change the inter-molecular spacing. In most real cases, however, the NI transition is characterized by the complex interplay between the average ionicity ρ on the molecular sites and the stack dimerization δ. The ionicity may act as an order parameter only in the case of discontinuous, first order phase transitions. While the inter-site Coulomb interaction V favors a discontinuous jump of ionicity, the intra-chain charge-transfer integral t mixes the fully neutral and fully ionic quantum states and favors continuous changes in ρ. The coupling of t to lattice phonons induces the dimerization of the stack, basically a Peierls-like transition to a ferroelectric state, which is a second order phase transition. Intramolecular (Holstein) phonons, on the other hand, modulate the on-site energy U and favor a discontinuous jump in ρ.