TMTTF Molecules Research Articles

Successive Dimensional Transition in (TMTTF)_{2}PF_{6} Revealed by Synchrotron X-ray Diffraction.

A quasi-one-dimensional organic charge-transfer salt (TMTTF)_{2}PF_{6} undergoes a multistep phase transition as the temperature decreases. One of these transitions is called a "structureless transition," and these detailed structures were unknown for many years. With synchrotron x-ray diffraction, we observed a slight structural difference owing to the effect of charge-order transition between two TMTTF molecules in a dimer, which corresponds to the charge transfer δ_{CO}=0.20e. The two-dimensional Wigner crystallization was determined from an electron density analysis using core differential Fourier synthesis. Furthermore, we found that the ground state due to tetramerization, called the spin Peierls phase, is a three-dimensional transition with interchain correlation.

Comprehensive Optical Investigations of Charge Order in Organic Chain Compounds (TMTTF)2X

Charge ordering in the (TMTTF)2X salts with centrosymmetric anions (X = PF‾6 , AsF‾6 , SbF‾6 ) leads to a ferroelectric state around 100 K. For the first time and in great completeness, the intra- and intermolecular vibrational modes of (TMTTF)2X have been investigated by infrared and Raman spectroscopy as a function of temperature and pressure for different polarizations. In this original paper, we explore the development and amount of charge disproportionation and the coupling of the electronic degrees of freedom to the counterions and the underlying lattice. The methyl groups undergo changes with temperature that are crucial for the anion cage formed by them. We find that the coupling of the TMTTF molecules to the hexafluorine anions changes upon cooling and especially at the charge-order transition, indicating a distortion of the anion. Additional features are identified that are caused by the anharmonic potential. The spin-Peierls transition entails additional modifications in the charge distribution. To complete the discussion, we also add the vibrational frequencies and eigenvectors based on ab-initio quantum-chemical calculations.

Anomalous Temperature Dependence ofg-Tensor in Organic Conductor, (TMTTF)2X (X=Br, PF6, and SbF6)

The magnetic properties of organic conductor (TMTTF) 2 X (X=Br, PF 6 , and SbF 6 ), where TMTTF is tetramethyltetrathiafulvalene, were examined by electron spin resonance (ESR) spectroscopy, X-ray diffraction (XRD) of the single crystals, and quantum–chemical calculation of the g -tensor. In the case of salts with bulky counter anions such as the PF 6 and SbF 6 , an anomalous temperature dependence of the g -tensor was observed in the temperature range from 20 to 296 K. This anomalous behavior of the g -tensor signifies the rotation of the principal axes as well as the shift of the principal values. The g -tensor of the Br salt is, however, temperature independent. No remarkable change in the intra-molecular structure as a function of temperature was observed for all salts. On the other hand, the distance between TMTTF and counter-anion molecules obviously decreases as the temperature decreases for the PF 6 and SbF 6 salts, while thermal contraction is not remarkable for the Br salt. In order to clarify the origin of the anomalous behavior of the g -tensor, we investigated the possibility of deformation of the wave-function by the counter-anion potentials using a quantum–chemical calculation for the actual crystal structures measured at low-temperatures. In this paper, we describe the first direct observation of the deformation of the frontier orbital by the counter anion potential for organic conductors. The intra-molecular spin-distribution as a function of temperature is also discussed from the microscopic point of view.

Crossover from Metal to Insulator and Charge Order Transition of (TMTTF)2SbF6 under Constant Pressure

Crossover from Metal to Insulator and Charge Order Transition of (TMTTF)₂SbF₆ under Constant Pressure

Charge and Anion Ordering in (TMTTF)2X Quasi-One-Dimensional Conductors

We present results of measurements of dielectric spectroscopy concerning the 4kF charge density wave associated with the charge ordering (CO) of Wigner type in (TMTTF)2X conductors. This CO transition is due to correlated electron interactions on the molecular chains and to the appropriate displacement of the anion chains (transition at q=0). Recent experiments on the substitution of hydrogen by deuterium in TMTTF molecules will be reported. In the vicinity of TCO it has also been found the divergence of the relaxation time and the development of a low frequency shoulder in the frequency dependence of the imaginary part of the dielectric permittivity, which are typical features for classical ferroelectrics.

不規則性をもつ結晶，超イオン導電体，非晶質，ガラス，融体の構造解析―不規則系・ランダム系の結晶学―１．周期系における不規則性の解析 有機擬一次元物質の秩序無秩序転移と電荷不均一化

High resolution X-ray structure analyses revealed the condition for the charge ordering (CO) observed in the quasi one-dimensional compounds (TMTTF) 2X. The size and orientational ordering of the counter anion X control the dimerization and tetramerization of TMTTF molecules. The molecular dimerization suppresses the 4kF CO, while the tetramerization assists the 2kF CO. The phase diagram of the electronic states including 2kF and 4kF CO has been proposed associated with magnitudes of the molecular dimerization and tetramerization.

4KFCDW induced by long range Coulomb interactions

By method of dielectric spectroscopy we have obtained experimental data concerning the 4k F charge density wave (CDW) associated with the charge ordering (CO) state of Wigner type in (TMTTF) 2 X conductors. A divergence of the relaxation time was found at the critical transition temperature T CO into the CO state. Below T CO a low frequency shoulder appears in the frequency dependence of the imaginary part of the dielectric permittivity. Under a moderate pressure, the peak in the real part of the dielectric permittivity, corresponding to the transition into the CO state, shifts to low temperature and broadens. The substitution of hydrogen by deuterium in TMTTF molecules results in an increase of T CO (isotopic effect). The results obtained confirm the ferroelectric character of the CO state; they also favour long range Coulomb interactions as the main driven force for the formation of this state.

Charge ordering in (TMTTF) 2X salts

We show that the newly discovered phase transitions in (TMTTF) 2X (X=PF 6, AsF 6, SbF 6, ReO 4) are characterised by a huge divergent peak in the low frequency dielectric constant. These phase transitions can be explained by combined effect of charge disproportionation on TMTTF molecules and the uniform shift of anions, yielding thus a macroscopic ferroelectric polarisation. Anion ordering transition, which occurs for non-centrosymmetrical anions, has a specific effect depending on the distortion vector: it destabilises the ferroelectric state in the (TMTTF) 2ReO 4 and brings an antiferroelectric character to the transition in (TMTTF) 2SCN.

NMR study of charge localized states of (TMTTF) 2Br

13C NMR study was performed for a quasi-one-dimensional organic conductor (TMTTF) 2Br, using a single crystal in which two central carbon sites of TMTTF molecules were labeled with 13C. To investigate the relation between the charge localization around 100 K and the magnetic ground state from the microscopic point of view, we measured the NMR spectra and nuclear relaxation rate. We found slight broadening of the spectra above the magnetic phase transition temperature (T N ∼15 K). We discuss this anomaly with the newly measured data of uniform susceptibility and ESR.

Enhanced charge localization in the organic alloys

Structural, electrical, and magnetic studies of [(TMTSF)[sub 1[minus][ital x]](TMTTF)[sub [ital x]]][sub 2]ReO[sub 4] alloys are reported. They exhibit around [ital x][similar to]0.5 an enhanced charge localization and a strong decrease of the (1/2,1/2,1/2) anion-ordering (AO) transition temperature. The localization enhancement is attributed to a 4[ital k][sub [ital F]] order of charge on the sites due to the alternate order of TMTSF and TMTTF molecules. This heteropolar charge distribution is suggested to weaken the 2[ital k][sub [ital F]] bond response and thus to depress the AO transition. These properties contrast with those exhibited by the hybrid salt (TMDTDSF)[sub 2]ReO[sub 4].

X-ray investigation of the tetramethyldithiadiselenafulvalene (TMDTDSF)2X series of organic conductors. I. Study of the orientational disorder of the TMDTDSF molecule

We present a study of the (TMDTDSF) 2 X materials, where TMDTDSF (Tetramethyldithiadiselenafulvalene) is a hybrid of the TMTSF and TMTTF molecules and X (PF 6 , AsF 6 , SbF 6 , ReO 4 , BF 4 and ClO 4 ) a monovalent anion. This paper deals with an X-ray diffuse scattering study of the static disorder of the orientation of non-centrosymmetrical TMDTDSF molecules. We present a calculation of the diffuse scattering by a model of disorder which takes into account the intrachain correlations between first neighbor molecules and the associated displacements (size effect). Computer simulations of the diffuse scattering intensity allows us to account for the experimental results. We find that in all the compounds investigated the two orientations of the molecules are not correlated and have equal probability in agreement with structural refinements. The interpretation of the diffuse scattering of (TMDTDSF) 2 PF 6 gives a size effect between first- and second-neighbor molecules of about 0.1A and 0.03 A respectively. As the orientational disorder is random, theses systems are good candidates for the study of the influence of a random disorder on the structural transitions of these materials such as the anion ordering and the Spin-Peierls instability

Studies on π-acceptor molecules containing the dicyanomethylene group. X-Ray crystal structure of the charge-transfer complex of tetramethyltetrathiafulvalene and 2,3-dicyano-1,4-naphthoquinone: (TMTTF)3–(DCNQ)2

The reaction of tetramethyltetrathiafulvalene (TMTTF, 7) with 2-dicyanomethyleneindane-1,3-dione (DCID, 1) leads to isomerisation of DCID to 2,3-dicyanonaphthoquinone (DCNQ, 3) to yield the charge-transfer complex [(TMTTF)3]2˙+–(DCNQ˙–)28. The X-ray crystal structure of complexe 8 reveals stacks of TMTTF molecules along the xdirection in the sequence A ‥ B ‥ A ‥ A ‥ B ‥ A ‥etc.. The A ‥ A and A ‥ B interplanar separation distances are 3.51 Å and 3.44 Å, respectively. The DCNQ radical anions form dinners, with a remarkably short interplanar separation of 3.02 Å, which are orthogonal to the TMTTF stacks. The complex is diamagnetic. Other π-acceptor molecules containing the dicyanomethylene group, viz. compounds 2 and 10–12, have been prepared and their redox properties studied by cyclic voltammetry. Compound 2 rearranges to compound 4 on complexation with TTF 6. DCID derivatives 1and 2 react with a range of amines, by displacement of one cyano group, to yield enamines 15–19; the amphiphilic compound 18 forms Langmuir–Blodgett films which exhibit weak second harmonic generation.

Molecular orientation in conductive Langmuir-Blodgett films of a charge-transfer complex

The molecular orientation in Langmuir-Blodgett films of a 3:2 charge-transfer complex of tetramethyltetrathiafulvalene and tetradecyltetracyanoquinodimethane [(TMTTF) 3(C 14TCNQ) 2] is discussed from the observed anisotropic ESR spectra at the K and X bands. Anisotropy of g values shows the existence of in-plane preferential orientation of the complex with the pπ orbital axis and long axis of the TMTTF molecules parallel and perpendicular to the dipping direction of the substrate, respectively. The effect of aging on ESR spectra of the system is also reported. The decrease in the difference in linewidth at the K and X bands observed for the external magnetic field normal to the film plane indicates the reduction in the degree of misorientation of the molecules by aging.

Crystal Structures and Electrical Properties of Hexacyano-Butadiene (HCBD) Charge Transfer Complexes

Abstract We have prepared charge transfer (CT) single crystals comprising TTF derivatives and hexacyanobutadiene (HCBD), which is one of the strongest acceptors. TMTTF gave two crystals in different stoichiometry, one is 2:1 semiconductor and the other is 1:1 insulator. In the 2:1 complex, TMTTF molecules form face-to-face segregated columns with dimerized manner and HCBD lies perpendicular to the donor columns to form side-by-side segregated columns in the cavity produced by donor columns. The ground state of this complex is antiferromagnetic. The 1:1 complex has face-to-face uniform segregated columns and shows some phase transition at around 150K. TTMTTF forms a semi conductive 1:1 complex with uniform face-to-face segregated column.

The Crystal Structure of (TMTTF)2ReO4

Abstract The crystal of (TMTTF)2ReO4 is isomorphous with the other (TMTTF) 2X compounds(X=ClO4, PF6, Br,···). TMTTF molecules are stacked face-to face to form a zig-zag column. Although the structure of (TMTTF)2ReO4 closely resembles that of the analogous compound, (TMTSF)2ReO4, the loose intermolecular contacts in (TMTTF)2ReO4 suggest that the intermolecular interaction appears to be much weaker than that in (TMTSF)2ReO4.

Conductivity and thermopower studies of bis-tetramethyltetraselenafulvalenium hexafluorophosphide, bis-tetramethyltetrathiafulvalenium hexafluorophosphide, and their solid solutions,(TMTSF1−xTMTTFx)2PF6

The conductivity $\ensuremath{\sigma}$ and the thermoelectric power $S$ have been studied experimentally on the organic conductors bis-tetramethyltetraselenafulvalenium hexafluorophosphide [${(\mathrm{TMTSF})}_{2}$P${\mathrm{F}}_{6}$] and bis-tetramethyltetrathiafulvalenium hexafluorophosphide [${(\mathrm{TMTTF})}_{2}$P${\mathrm{F}}_{6}$] and their solid solutions: ${(\mathrm{TMTS}{\mathrm{F}}_{1\ensuremath{-}x}\mathrm{TMTT}{\mathrm{F}}_{x})}_{2}\mathrm{P}{\mathrm{F}}_{6}$. Dramatic effects are seen in $\ensuremath{\sigma}$ already when dilute concentrations of TMTTF molecules are introduced in the TMTSF chains, and for $x=0.1$, $\ensuremath{\sigma}$ shows generally activated behavior. The thermopower, on the contrary, remains basically unaffected for $x$ at least as large as 0.25. These unusual findings are attributed to the small transverse transfer integral ${t}_{b}$ associated with the TMTTF molecule, which leads to large on-site Coulomb repulsion. The parallel transfer integrals ${t}_{a}$ appear to be very similar for the two constituent molecules. A slight $x$ dependence of $S$, appearing below approximately 100 K, is attributed to enhanced one-dimensionality when the amount of TMTTF is increased. Close to and below the spin-density-wave ordering temperature, more marked $x$ dependences appear, indicative of impurities. The $x=0.85$ salt behaves generally similar to the pristine TMTTF material, though dominated by additional impurity levels.

Tetramethyltetrathiafulvalene iodides and bromides

1. We prepared some tetramethyltetrathiafulvalene (TMTTF) iodides and bromides of variable composition. 2. The TMTTF iodides and bromides, which contain a conducting stack composed of TMTTF molecules with a partial charge on them smaller than one, have high electrical conductivity values, in contrast to compounds with a complete transfer of the charge.