Complex changes in structural parameters hidden in the universal phase diagram of the quasi-one-dimensional organic conductors (TMTTF)2X ( X=NbF6, AsF6, PF6 , and Br) (original) (raw)
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Magnetism and Charge ordering in TMTTF $ _2 −PF-PF −PF _6 $ organic crystals
Arxiv preprint arXiv: …, 2011
Using a combination of Density Functional Theory, mean-field analysis and exact diagonalization calculations we reveal the emergence of a dimerized charge ordered state in TMTTF2-PF6 organic crystal. The interplay between charge and spin order leads to a rich phase diagram. Coexistence of charge ordering with a structural dimerization results in a ferroelectric phase, which has been observed experimentally. The tendency to the dimerization is magnetically driven revealing TMTTF2-PF6 as a multiferroic material. PACS numbers: 71.30.+h, 71.45.Lr, 71.20.Rv In recent years, organic molecular crystals have emerged as a fascinating class of materials with immense potential for applications. Structures in these materials are held together by van der Waals and hydrogen bonding as opposed to much stronger covalent and ionic bonding in conventional crystals. As a consequence, these systems present complex phases and phenomena which still require a microscopic understanding, ranging from anomalous positive and negative thermal expansion[1] to hightemperature ferroelectricity with large polarization[2] and even superconductivity . Moreover, their light, flexible and nontoxic character makes them ideal candidates for future applications.
Structural investigation of the (TMDTDSF)2X series of organic conductor
Synthetic Metals, 1991
We present an X-ray study of a new series of organic conductors (TMDTDSF)2X based on an hybrid molecule of TMTSF and TMTTF. An analysis of the Laue diffuse scattering shows that in crystals with small anions X like CIO4, BF4, PF 6 and AsF 6, the unsymmetrical molecule TMDTDSF is orientationaly disordered, in contrast with (TMDTDSF)2ReO 4 where a weak disorder is found. The temperature behavior of (TMDTDSF)2ReO 4 and (TMDTDSF)2BF 4 is then presented. We show that in the former compounds the anion ordering (A.O.) transition still occurs at the q1=(1/2,1/2,1/2) reduced wave vector, at a temperature Tc=165 K intermediate between that of the A.O. transitions of (TMTSF)2ReO 4 (177 K) and of (TMTTF)2ReO 4 (154. K). In the BF 4 salt a local ql A.O. appears below ~22 K instead of the transitions observed at 36 K and 40 K in the Se-and S-based salts respectively. This reveals the essential r61e of the chains of molecules in these ql A.O. transitions.
Structural investigation of new 2 : 1 series of organic conductors
Synthetic Metals, 1991
We report on the relation between the structural and physical properties of 3 new series of 2 : 1 salts isomorphous to the Bechgaard salts, but based on the unsymmetrical molecules BCPTTF, tTTF and TMDTDSF. In the two first series the orientation of the molecules is ordered. With octahedral anions, (BCPTTF)2X and (tTTF)2X exhibit respectively spin-Peierls and antiferromagnetic ground states at critical temperatures sizeably higher than for the (TMTTF)2X series. In (TMDTDF)2X when the anions are small the orientation of the TMDTDSF molecule is found to be disordered. This disorder has dramatic effects on the anion ordering (A.O.) transition occuring when X is non-centrosymmetric. These effects are analyzed in the light of our present understanding of the A.O. mechanism and compared with previous results obtained with the (TMTSF)2_ x (TMTTF)x CIO 4 and (TMTSF)2 (CIO4) l_x (ReO4)x solids solutions.
Journal of Structural Chemistry, 2009
We analyze the specific features of the structure and properties of crystals that belong to the family of lowdimensional molecular conductors based on BEDT-TTF (ET) radical cation salts with [MNOX 5 ] 2-(M = Os, Ru; X = Cl, Br) mononitrosyl metal complexes as anions. It is shown that when the synthesis components are changed single crystals with , , , and -types of the conducting layers form. All radical cation salts of this family can be considered as pseudopolymorphous phases with a general formula of (ET) 4 [MNOX 5 ] 2-x G x (G is a guest solvent molecule, BN or NB). The studied crystals display a range of remarkable structural features such as conformational and charge ordering in the conducting ЕТ layers as well as commensurate and incommensurate structural modulations mainly related to the positional ordering of the components of complex anion layers.
2005
We present a detailed study of the Nernst effect Nzx in (TMTSF)2PF6 as a function of temperature, magnetic field magnitude and direction and pressure. As previously reported there is a large resonant-like structure as the magnetic field is rotated through crystallographic directions, the Lebed Magic Angles. These Nernst effect resonances strongly suggest that the transport of the system is effectively "coherent" only in crystallographic planes along or close to the applied field direction. We also present analytical and numerical calculations of the conductivity and thermoelectric tensors for (TMTSF)2PF6, based on a Boltzmann transport model within the semi-classical approximation. The Boltzmann transport calculation fails to describe the experiment data. We suggest that the answer may lie in field induced decoupling of the strongly correlated chains.
Infrared studies of charge ordering in quasi one-dimensional organic conductors
2004
Interactions between charge, spin, or lattice degrees of freedom result in the extraordinary rich phase diagram of the quasi one-dimensional (TMTTF)_2X and (TMTSF)_2X charge-transfer salts with ground states like charge order, spin order, anion order, and superconductivity. The focus of this contribution is on the charge-order transition in (TMTTF)_2AsF6 and (TMTTF)_2PF6 which is related to charge disproportionation along the molecular stacks at moderate temperatures T_CO. We investigated this phase transition into a ferroelectric state by infrared spectroscopy. For the first time, the lattice dynamics was explored in the vicinity of T_CO. We observed a splitting of the electron-molecular-vibration coupled totally symmetric intramolecular a_g(ν_3) mode in the charge-ordered state. From this result a charge disproportionation on the TMTTF molecular stacks of about 2:1 in (TMTTF)_2AsF6 and 1.25:1 in (TMTTF)_2PF6 can be estimated. Below T_CO, we found in both compounds new absorption modes in the conductivity spectrum at very low frequencies ν < 100 cm-1.
Mid-infrared response of charge-ordered quasi-1D organic conductors (TMTTF) 2 X
Journal de Physique IV (Proceedings), 2005
We investigated the temperature dependence of the charge disproportionation in the charge ordered quasi one-dimensional conductors (TMTTF) 2 AsF 6 and (TMTTF) 2 PF 6 by infrared spectroscopy. In the mid-infrared, we observed a splitting of characteristic vibronic modes below the charge-order transition which can be directly linked to the charge disproportionation on the molecules. From our analysis of these modes we extracted the temperature dependence of the charge disproportionation which is much stronger in (TMTTF) 2 AsF 6 compared to (TMTTF) 2 PF 6. The charge disproportionation is only weakly affected by the spin-Peierls transition in the AsF 6 salts and remains basically unchanged in the PF 6 salts. Therefore, our data confirm that the spin-Peierls and charge-ordered ground state coexist at low temperatures.
Chemistry of Materials, 2008
Electrochemical oxidation of 4,5-ethylenedioxy-4′-methyl-tetrathiafulvalene (MeEDO-TTF) afforded three types of (MeEDO-TTF) 2 PF 6 radical cation salts under different conditions: black powder, dark green plates, and black plates. The optical absorption spectra of these modifications were very similar to each other. The black powder and dark-green plate modifications were semiconducting and metallic, respectively, and the latter showed a semiconducting behavior below 200 K. The black plate modification exhibited a semiconductor-to-semiconductor first-order phase transition at around room temperature (303 K). The donor molecules in both the high-and low-temperature phases of this modification showed similar packing patterns, based on which the tight-binding approximation afforded similar two-dimensional Fermi surfaces. The phase transition of this modification is accompanied by a subtle change in the relative orientation of the neighboring donor molecules. The vibrational spectra proved that a nearly localized charge disproportionation takes place in the high-temperature phase and the distinct charge disproportionation is developed in the low-temperature phase. These results indicate that the slight lattice distortion assists the stabilization of charge disproportionation. The comparison of the isostructural salts of metallic (MeEDO-TTF) 2 X (X ) BF 4 , ClO 4 ) and the semiconducting high-temperature phase of the (MeEDO-TTF) 2 PF 6 demonstrated that the side-by-side intermolecular interaction is most effectively modulated according to the anion size. The unit cell parameter corresponding to this intermolecular interaction was varied only ca. 2% among these isostructural salts. This observation demonstrates the very sensitive nature of the transport property to the lattice modulation. (a) Williams, J. M.; Ferraro, J.; Thorn, R. J.; Carlson, K. D.; Geiser, U.; Wang, H.-H.; Kini, A. M.; Wangbo, M. H. Organic Superconductors (including Fullerenes); Prentice Hall: Englewood Cliffs, NJ, 1992. (b) Ishiguro, T.; Yamaji, K.; Saito, G. Organic Superconductors, 2nd ed.; Springer-Verlag: Berlin, 1998.
One dimensional physics in organic conductors (TMDTDSF)2X, susceptibility and 77Se-NMR experiments
Synthetic Metals, 1993
We present an analysis of spin susceptibility and 77Se nuclear relaxation data for two members of the mixed S-Se series (TMDTDSF)2X, X=ReO4 and PF6. For both compounds, the temperature dependence of the relaxation enhancement ('IT1)-1 follows, in the paramagnetic regime, the square of the temperature dependent uniform static susceptibility measured separately both by ESR and Faraday balance techniques. Then a phase transition driven by anion ordering is evidenced for the ReO 4 salt. However, deviations from the high temperature behaviour are visible in the PF6 salt below the charge localisation temperature and are explained in terms of antiferromagnetic correlations. The scaling theory for the quasi-one-dimensional (Q-I-D) electron gas model accounts very well for the NMR behaviour of these mixed molecule systems.
Transport properties and phase transition of organic crystal βd′ -(BEDT-TTF)2I3
Synthetic Metals, 1989
A peculiar resistivity anomaly has been observed in a new phase of (BEDT-TTF)2I 3 first synthesized recently by Zhu et al. using a diffusion method. The structure of the new phase is similar to that of its isomer ~-(BEDT-TTF)213, with a slightly different arrangement of iodine atoms and lattice constants. But, in contrast to the H-phase, which remains metallic when the temperature decreases and finally becomes a superconductor, the new phase undergoes a metal-semiconductor transition at about 140 K. Then, after a rapid rise, the resistivity peaks at ~60 K. However, the thermopower measurement shows only a small anomaly at the phase transition. A model with two energy bands is assumed to explain these phenomena. The quite different behaviour of the new phase seems to imply that the iodine anion plays a subtle role in the determination of the properties of the material.