M. H. Lemée-Cailleau

Structural aspects of the neutral-to-ionic transition in an organic charge-transfer crystal: TTF-chloranil

Abstract The structural aspects of the neutral-to-ionic transition in TTF-CA are discussed in relation with recent results on neutron scattering studies, observation of metastable behaviour around the transition and determination of the phase diagram.

Neutron and raman scattering study of the soft mode behaviour at the structural phase transition in A-tCNB

Abstract Direct observations of the soft mode in A-TCNB by inelastic neutron scattering in association with Raman scattering are presented. These measurements indicate a complex dynamic regime which seems to be intermediate between displacive and order-disorder types.

Structural Properties, Dynamics and Pressure Effects at the Neutral-to-Ionic Transition

Mixed-stack organic charge-transfer complexes which undergo a neutral-to-ionic transition are model systems to study strong cooperative effects between lattice-relaxed electronic excitations. An overview is given on structural and dynamical properties associated with this electronic-structural phase transformation which can be photo-induced. The analysis of the structural changes allows to underline the respective parts taken by the ionicity, which is a totally symmetric order parameter, and the dimerization which is the symmetry-breaking order parameter. The cooperative mechanism and the thermodynamics of the neutral-to-ionic transition are discussed on the basis of pressure effects, in particular with the observation of the tendency towards a tricritical point at high-pressure. This uncommon phase transition can be viewed as resulting from the condensation and the ordering of ionic dimers. However it needs to take into account quantum effects which are of great importance. Two types of characteristic excitation exist, on the one hand a polar vibrational soft mode of dimerization, on the other hand lattice-relaxed charge-transfer excitations which can be thermally- or photo- induced. Pretransitional dynamics is discussed in order to give a consistent view on the different results, recent and earlier.

Crossover of the dynamical regime of the spt in A-TCNB under hydrostatic pressure

Abstract Anthracene-TCNB is a weak charge-transfer molecular crystal which presents a second order phase transition. At atmospheric pressure, a soft mode has been observed by inelastic neutron scattering in the high temperature phase. Under hydrostatic pressure, the transition remains continuous whereas a drastic change occurs in the dynamical regime. At 3 kbar, at the superlattice point, the soft mode-type scattering is indeed replaced by a quasielastic response. This feature indicates a continuous crossover from a displacive regime to an order-disorder regime for a modest pressure of 1.5 kbar.

Raman scattering study under pressure at 6 K of the N = 4 incommensurate phase II of biphenyl

Abstract Pressure lowers the domain of existence of the incommensurate phase II of biphenyl. This allows a study of the characteristic dynamics of an incommensurate phase with n = 4 in optimal conditions i.e. at very low temperature. Raman scattering in such extremal conditions have allowed to give complementary results with respect to previous neutron scattering experiments and so to extend the knowledge of the characteristic phason and amplitudon branches by investigating a larger range of frequency with a good resolution.

CCDC 604869: Experimental Crystal Structure Determination

Related Article: M.Le Cointe, M.H.Lemee-Cailleau, H.Cailleau, B.Toudic, L.Toupet, G.Heger, F.Moussa, P.Schweiss, K.H.Kraft, N.Karl|1995|Phys.Rev.B|51|3374|doi:10.1103/PhysRevB.51.3374

Physics of Neutral-to-Ionic Phase Transition in Organic Charge Transfer Semiconducting Compounds

An uncommon excitonic instability takes place in some exotic semiconducting compounds. Indeed, the equilibrium neutral-to-ionic (N-I) phase transition, as well as the non-equilibrium photo-induced phase transforma tion, observed in some organic charge-transfer complexes, originate from intraand inter-chain cooperative effects between structurally relaxed charge-transfer excitations. This electronic-structural phase transition manifests itself by a change of the degree of charge-transfer and a dimerization distortion with the formation of donor-acceptor pairs along the stacking axis in the I phase. Thermal charge-transfer excitations associated with the formation of I strings along N chains are at the heart of the mechanism of this phase transition. These relaxed electronic excitations, which are an intrinsic feature of low-dimensional systems with strong electron-phonon coupling, can be described in terms of self-trapping and self-multiplication of charge-transfer excitons. Precise structural studies on the prototype compound, tetrathiafulvalene-p-chloranil allow to highlight the respective pole taken by the ionicity and the dimerization. Symmetry and thermodynamics analysis of the N-I transition, based on recent determination of the pressure-temperature phase diagram, make possible to present a consistent picture of this phase transition. Supported by theoretical considerations taking into account the interplay between quantum and thermal effects, the experimental observations show that the N-I transition results from the condensation and the ordering (crystallization) of charge-transfer excitations, following a phase diagram analogous to the solid—liquid—gas one.