One of the most fascinating features of some organic materials is the possibility to tune between different ground states. This situation especially occurs in some quasi-one-dimensional charge-transfer (CT) complexes with a mixed-stack architecture, where the alternation of electron donor (D) and electron acceptor (A) molecules gives rise to chain multistability between a regular neutral (N) state ...D°A°D°A°D°A°... and two degenerate dimerised ionic (I) states ...(D+A­)(D+A­)(D+A­)... and ...D+)(A­D+)(A­D+)(A­... At finite temperature the thermal excitation of boundaries between "phases" destroys any long range order and the isolated chain becomes intrinsically inhomogeneous with the condensation of structurally relaxed CT exciton-strings. Inter-chain coupling in the crystal can drive phase transitions [1], and thus the so-called N-I transition may be induced by temperature, pressure and also light. Crystalline interchain organisations may lead to different ordering schemes, such as the interesting possibility for this transition to occur in stages ("staging") due to frustrated interstack Coulomb interactions, when, between certain planes, each D (or A) has other D (or A) nearest neighbours [2]. It has been theoretically predicted that N layers may be inserted between I layers to reduce the Coulomb repulsion, so that multiple periodic long range ordering may occur in the ground state within an infinite stepped sequence of first order transitions (Devil's staircase). It is of fundamental interest to obtain some experimental evidences of such complex charge ordering phenomena due to long range Coulomb and electron-lattice interactions.

We have performed high resolution X-ray scattering measurements on BM2 (the D2AM CRG beamline) on the 2,6-dimethyltetrathiafulvalene- p-chloranil (DMTTF-CA) which appears from spectroscopic measurements as a good candidate. We have observed below Tc = 65.5 K, and in a direction perpendicular to the stack axis, the emergence of superstructure Bragg peaks characteristic of a cell-doubling. This constitutes the first direct structural evidence of periodic ordering of neutral and ionic planes. The structural analysis has shown that simultaneously to the neutral-ionic layer ordering, theoretically expected, an antiparallel dimerisation ordering takes place, leading to an unconventional "ferrielectric" structure (Figure 64).

An important physical feature of the observed phenomenon is the interplay between the staging ordering and the CT exciton-string condensation. Thus, instead of a multistep behaviour, only one cell-doubling occurs related to the largest plateau of the Devil's staircase with a continuous evolution of the order parameter, as evidenced by the temperature evolution of superstructure peak intensity. Above Tc, diffuse quasi-planes perpendicular to the stacking axis are clearly observed (Figure 65). This is a direct signature of the quasi-one dimensional nature of these compounds and gives the first direct estimation of the size of condensed-relaxed CT exciton-strings (more than 100 Å). These non-linear excitations are at the heart of the unusual physical properties of these systems, in particular of the photo-induced phase transformations due to cooperative electron transfer, which has begun to be studied by ultrafast time-resolved crystallography on ID9 beamline.

References
[1] M.H. Lemée-Cailleau, M. Le Cointe, H. Cailleau, T. Luty, F. Moussa, J. Roos, D. Brinkmann, B. Toudic, C. Ayache, N. Karl, Phys. Rev. Lett., 79, 1690-1693 (1997).
[2] J. Hubbard, J.B. Torrance, Phys. Rev. Lett, 47, 1750-1754 (1981) and R. Bruinsma, P. Bak, J.B. Torrance, Phys. Rev. B, 27, 456-466 (1983).

Principal Publication and Authors
E. Collet (a), M. Buron-Le Cointe (a), M.H. Lemée-Cailleau (a), H. Cailleau (a), M. Meven (b), S. Mattauch (b), G. Heger (b), J.F. Bérar (c), N. Karl (d), Phys. Rev. Lett, submitted.

(a) Université Rennes 1, GMCM, UMR-CNRS 6626, Rennes (France)
(b) RWTH, Institut für Kristallographie, Aachen (Germany)
(c) ESRF and CNRS, Grenoble (France)
(d) Universität Stuttgart, Physikalisches Institut (Germany).