Pascale Foury-Leylekian

Charge ordering, symmetry and electronic structure issues and Wigner crystal structure of the quarter-filled band Mott insulators and high pressure metals δ-(EDT-TTF-CONMe2)2X, X = Br and AsF6

We report on the synthesis and application of an internal chemical pressure to effectively control, and reduce, the Mott gap in the system δ-(EDT-TTF-CONMe2)2X, X = Br, AsF6; the detailed accounts of its Pmna, averaged room temperature structure and reversible phase transition at ca. 190 K towards a low temperature P21/a structure; the synthesis of (13C-EDT-TTF-CONMe2)2Br, where one carbon atom of the inner double bond is 100% 13C-enriched and single crystal 13C solid state NMR spectroscopy and relaxation revealing that charge ordering occurs at room temperature and ambient pressure; the discovery of weak superstructure Bragg reflections in δ-(EDT-TTF-CONMe2)2Br and subsequent analysis of the superstructure symmetry and refinement of an exhaustive synchrotron radiation data set; suggesting an alternation at room temperature of neutral and oxidized molecules along both the stacking a and transverse b directions in orthorhombic, non-centrosymmetric space groupP2nn, a CO pattern compatible with ferroelectricity. The charge disproportionation and long range order crystallization of the electron gas onto every other molecular site within a three-dimensional Wigner lattice is coupled to a concerted activation-deactivation of large collections of transverse Csp2–H⋯O hydrogen bonds and an anti-phase, static modulation of the bromide anions displacements along b. Despite the occurrence of charge ordering, the stacks remain essentially uniform, in agreement with the rich low temperature Mott physics of the system.

Multistability in a family of DT–TTF organic radical based compounds (DT–TTF)4[M(L)2]3 (M = Au, Cu; L = pds, pdt, bdt)

The preparation of new charge transfer salts based on the DT–TTF donor and monoanionic square planar transition metal bisdichalcogenide complexes M(L)2 (M = Au, Cu; L = pds, pdt, bdt, where pds = pyrazine-1,2-diselenolate, pdt = pyrazine-1,2-dithiolate, bdt = benzene-1,2-dithiolate) are reported and these salts are characterized by X-ray diffraction, EPR spectroscopy, electrical conductivity, thermoelectric power and static magnetization measurements, as well as tight-binding band structure calculations. Three compounds, (DT–TTF)4[AuIII(bdt)2]3, (DT–TTF)4[CuIII(pds)2]3, (DT–TTF)4[CuIII(pdt)2]3, are members of a general family of compounds of DT–TTF with stoichiometry 4 : 3 as the previously reported (DT–TTF)4[AuIII(pds)2]3. Although not strictly isostructural, all members of this family present a similar crystal packing motif of the donor and acceptor units and present a common pattern of first- and second-order phase transitions as seen in electrical transport and magnetic properties. The second-order phase transition is ascribed to a variation from dynamic to static charge ordering. With [CuIII(pdt)2] and DT–TTF a 1 : 1 salt with formula (DT–TTF)[CuIII(pdt)2] was also obtained. The structural differences and similarities between the four compounds (DT–TTF)4[M(L)2]3 (M = Au, Cu; L = pds, pdt, bdt), that are related to their physical properties, and the reversible phase transitions observed are discussed.

Low-temperature structural effects in the (TMTSF)2PF6 and AsF6 Bechgaard salts

We present a detailed low-temperature investigation of the statics and dynamics of the anions and methyl groups in the organic conductors (TMTSF)2PF6 and (TMTSF)2AsF6 (TMTSF: tetramethyl-tetraselenafulvalene). The 4 K neutron-scattering structure refinement of the fully deuterated (TMTSF)2PF6-D12 salt allows locating precisely the methyl groups at 4 K. This structure is compared to the one of the fully hydrogenated (TMTSF)2PF6-H12 salt previously determined at the same temperature. Surprisingly, it is found that deuteration corresponds to the application of a negative pressure of 5 × 10 2 MPa to the H12 salt. Accurate measurements of the Bragg intensity show anomalous thermal variations at low temperature both in the deuterated PF6 and AsF6 salts. Two different thermal behaviors have been distinguished. Small Bragg-angle measurements reflect the presence of low-frequency modes at characteristic energies θE = 8.3 K and θE = 6.7 K for the PF6-D12 and AsF6-D12 salts, respectively. These modes correspond to the low-temperature methyl group motion. Large Bragg-angle measurements evidence an unexpected structural change around 55 K, which probably corresponds to the linkage of the anions to the methyl groups via the formation of F ... D-CD2 bonds observed in the 4 K structural refinement. Finally we show that the thermal expansion coefficient of (TMTSF)2PF6 is dominated by the librational motion of the PF6 units. We quantitatively analyze the low-temperature variation of the lattice expansion via the contribution of Einstein oscillators, which allows us to determine for the first time the characteristic frequency of the PF6 librations: θE ≈ 50 K and θE = 76 K for the PF6-D12 and PF6-H12 salts, respectively.

Exploring the charge-ordering transition in (TMTTF)2X via thermal expansion measurements

Abstract We report results of high-resolution measurements of the c * ‐axis expansivity ( α c * ) at the charge-ordering (CO) transition for the quasi-1D (TMTTF) 2 X compounds with X = SbF 6 and Br and make a comparison with previous results for the X = PF 6 and AsF 6 salts. For X = SbF 6 , due to the screening of the long-range Coulomb forces, a sharp λ -type anomaly is observed at T CO , which contrasts with the step-like mean-field anomaly at T CO for PF 6 and AsF 6 , where CO occurs in the Mott–Hubbard charge-localized regime. For the latter two salts, a negative contribution to α c * is observed above T CO . This feature is assigned to the anions’ rigid-unit modes, which become inactive for T T CO . Our α c * results for the X = Br salt, where such rigid-unit modes are absent, reveal no traces of such negative contribution, confirming the model based on the anions’ rigid-unit modes for the X = PF 6 and AsF 6 salts.

Donor–anion interactions at the charge localization and charge ordering transitions of (TMTTF)2AsF6 probed by NEXAFS

High-resolution near-edge X-ray absorption fine structure (NEXAFS) measurements at the As M-edge, F K-edge and S L-edge of the Fabre salt (TMTTF)2AsF6 were performed from room temperature (RT) to 90 K, allowing to reach the charge localization regime below Tρ ≈ 230 K and to cross the charge ordering (CO) transition at TCO ≈ 102 K. The F K-edge and S L-edge spectra exhibit several transitions which have been indexed on the basis of first-principles DFT calculations. Upon cooling from RT significant energy shifts up to +0.8 eV and -0.4 eV were observed in transitions exhibited by the F 1s and S 2p spectra respectively, while the As 3p doublet does not show a significant shift. Opposite energy shifts found in the F 1s and S 2p spectra reflect substantial thermal changes in the electronic environment of F atoms of the anion and S atoms of TMTTF. The changes found around the charge localization crossover suggest an increase of the participation of the S d orbitals in the empty states of TMTTF as well as an increase of the strength of donoranion interactions. A new F 1s pre-edge signal detected upon entry into the CO phase is a clear fingerprint of the symmetry breaking occurring at TCO. We propose that this new transition is caused by a substantial mixing between the HOMO of the AsF6(-) anion and the unoccupied part of the TMTTF HOMO conduction band. Analysis of the whole spectra also suggests that the loss of the inversion symmetry associated with the CO is due to an anion displacement increasing the strength of SF interactions. Our data show unambiguously that anions are not, as previously assumed, innocent spectators during the electronic modifications experienced by the Fabre salts upon cooling. In particular the interpretation of the spectra pointing out a thermally dependent mixing of anion wave functions with those of the TMTTF chains demonstrates for the first time the importance of anion-donor interactions.

Ground state of the quasi-1D compound BaVS3 resolved by resonant magnetic x-ray scattering.

Resonant magnetic x-ray scattering near the vanadium L2,3-absorption edges has been used to investigate the low temperature magnetic structure of high quality BaVS3 single crystals. Below T(N)=31  K, the strong resonance revealed a triple-incommensurate magnetic ordering at the wave vector (0.226   0.226   ξ) in hexagonal notation, with ξ=0.033. The azimuthal-angle dependence of the scattering signal and time-dependent density functional theory simulations indicate an antiferromagnetic order within the ab plane with the spins polarized along a in the monoclinic structure.

Microwave dielectric study of spin-Peierls and charge-ordering transitions in ( TMTTF ) 2 PF 6 salts

We report a study of the 16.5 GHz dielectric function of hydrogenated and deuterated organic salts (TMTTF)

Charge ordering in substituted and non-stoichiometric BaVS3

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Resonant inelastic x-ray scattering probes the electron-phonon coupling in the spin liquid kappa-(BEDT-TTF)2Cu2(CN)3

PF

Magnetoelastic coupling in the spin-Peierls ground state of hydrogenated and deuterated (TMTTF)2PF6salts

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