TMTSF Salts Research Articles

Crystal Structures and Electrical Resistivity of Three Exotic TMTSF Salts with I 3 − : Determination of Valence by DFT and MP2 Calculations

Three novel organic conductors (TMTSF)8(I3)5, (TMTSF)5(I3)2, and (TMTSF)4(I3)4·THF (THF = tetrahydrofuran) were synthesized and their crystal structures were characterized by X-ray diffraction analyses, where TMTSF denotes tetramethyltetraselenafulvalene. The crystals of both the (TMTSF)8(I3)5 and (TMTSF)5(I3)2 are composed of one-dimensional stacks of TMTSF trimers separated by TMTSF monomers. The crystal of the (TMTSF)4(I3)4·THF is composed of the TMTSF tetramers and I3− tetramers; and regarded as the elongated rock-salt structure. The electrical conductivity of the (TMTSF)8(I3)5 and (TMTSF)5(I3)2 is about 60 and 50 S·cm−1 at room temperature, respectively. The electrical resistivity of (TMTSF)8(I3)5 is weakly metallic below room temperature and rapidly increases below 88 and 53 K on cooling suggesting two possible phase transitions. The electrical resistivity of (TMTSF)5(I3)2 is semiconducting below room temperature but shows an anomaly around 190 K, below which the activation energy becomes small. The application of hydrostatic pressure up to 1.7 GPa do not change these behaviors of (TMTSF)8(I3)5 and (TMTSF)5(I3)2 very much. A method to evaluate the non-integer valence of crystallographically independent TMTSF molecules is developed by using the DFT (density-functional-theory) and MP2 (Hartree–Fock calculations followed by Møller–Plesset correlation energy calculations truncated at second order) calculations. It is shown that the method gives the valence of the TMTSF molecules of the I3 salts consistent with their electrical properties.

Role of anion ordering and effective pressure in the field-induced spin-density-wave phase of (TMTSF)2X

Magnetoresistance and Hall resistance measurements were conducted in the field-induced spin-density-wave (FISDW) phase of (TMTSF)2ReO4 above 1.0 GPa, with an anion ordering specified by . The quantized Hall resistance shows the sequence , with decreasing field that is successfully explained by the “extended standard model”. Consequently, we demonstrate that the difference between the chemical and hydrostatic pressures is linked to the appearance of the peculiar FISDW phase of the TMTSF salts with .

Novel superconducting phenomena in quasi-one-dimensional Bechgaard salts

It is the saturation of the transition temperature Tc in the range of 24 K for known materials in the late sixties which triggered the search for additional materials offering new coupling mechanisms leading in turn to higher Tc's. As a result of this stimulation, superconductivity in organic matter was discovered in tetramethyl-tetraselenafulvalene-hexafluorophosphate, (TMTSF)2PF6, in 1979, in the laboratory founded at Orsay by Professor Friedel and his colleagues in 1962. Although this conductor is a prototype example for low-dimensional physics, we mostly focus in this article on the superconducting phase of the ambient-pressure superconductor (TMTSF)2ClO4, in which the superconducting phase has been studied most intensively among the TMTSF salts. We shall present a series of experimental results supporting nodal d-wave symmetry for the superconducting gap in these prototypical quasi-one-dimensional conductors.

Raman studies of TMTSF salt of cobalt bis(dicarbollide) anion

Spectral investigations of the new organic semiconductor of the formula (TMTSF) 2[3,3′-Co(1,2-C 2B 9H 11) 2], were performed using Raman and quantum chemical simulation methods. The polarized Raman spectra of single crystals of the salt were recorded in the range 150–3100 cm −1. An assignment of the strongest vibrational Raman lines was proposed. The possibility of the presence of two rotamers of the Co anion has been discussed taking into account the DFT calculations and the experimental spectra of the reference material.

New organic magnetic anions (n=0–3 for A=H, n=2 for A=CH3) and their TTF, TMTSF, and/or BEDT-TTF salts

Five TEMPO-containing anions TEMPO-CONA(CH2)nSO3- (H0−, A = H, n = 0; H1−, A = H, n = 1; H2−, A = H, n = 2; H3−, A = H, n = 3; M2−, A = CH3, n = 2) have been prepared as PPh4 salts. So far three TTF salts ((TTF)(H1) · 2H2O, (TTF)(H2) · H2O, and (TTF)(M2) · 0.50H2O), one TMTSF salt ((TMTSF)(H1) · 2H2O · PhCl), and one BEDT-TTF salt (α-(BEDT-TTF)5(H1)2 · 6H2O) have been obtained. Their crystal structures, electrical resistivities, and magnetic properties are reported.

Quasi-one-dimensional electronic structure of(DMET)2CuCl2

The anisotropic transfer integrals characterizing the quasi-one-dimensional (Q1D) electronic structure are determined experimentally for the new organic superconductor ${(\mathrm{DMET})}_{2}{\mathrm{CuCl}}_{2}$ by angle-dependent magnetoresistance and polarized reflectance measurements. Lebed resonances and third angle effect are observed in the angle-dependent magnetoresistance; the latter gives the ratio of the inter- and intra-stack transfer integrals as ${t}_{{a}^{\ensuremath{'}}}∕{t}_{b}=0.1$. From the anisotropy of the plasma frequency found in the polarized reflectance spectra, the intra- and inter-stack transfer integrals are deduced as ${t}_{b}=0.28\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and ${t}_{{a}^{\ensuremath{'}}}=0.028\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, respectively, in good agreement with the results of the third angle effect. The observed anisotropy of the transfer integrals is the same as those for the Q1D TMTSF salts, which is consistent with the appearance of the field-induced spin-density-wave states at magnetic fields similar to those of the Q1D TMTSF salts.

A new quantum Hall effect in the two-dimensional organic conductor, τ-(EDO-S,S-DMEDT-TTF)2(AuBr2)1+y

In the two-dimensional organic conductor, τ-(EDO-S,S-DMEDT-TTF)2(AuBr2)1+y, we have observed Shubnikov de Hass oscillations with Landau level down to n=2 in field up to 27 T. Motivated with this result, we extended to Hall effect study in pulsed magnetic field up to 60 T as well as in dc field up to 45 T and found Hall resistance plateau above 40 T (n=1). Since this system consists of two two-dimensional Fermi surface pockets, and larger pocket has a large effective mass compared with the smaller one, larger Fermi pocket behaves as a reservoir for the smaller pocket to be well-separated into completely filled and completely empty Landau levels in a certain range of temperature and magnetic field. The interpretation of realizing the Hall plateau can be understood as a new mechanism for QHE, but is quite different neither from localization nor field induce spin density wave state in organic TMTSF salts.

Theoretical studies on magnetic interactions in many types of organic donor salts: BEDT-TTF, BETS, TMTTF and TMTSF

Intermolecular magnetic interaction of some donors, i.e. BEDT-TTF, BETS, TMTTF and TMTSF, in some organic superconductive crystals were studied theoretically by using ab initio MO and DFT methods. In addition, donor arrangements in both κ- and λ-phases crystals are also focused. For this purpose we employed κ-BETS2FeCl4 and λ-BETS2FeCl4 crystals for BETS salts; κ-(BEDT-TTF)2Cu[N(CN)2]X (X=Cl, Br and I) and κ-(BEDT-TTF)2Cu(NCS)2 crystals for BEDT-TTF salts; TMTTF2PF6 and TMTSF2PF6 crystals for TMTTF and TMTSF salts. For these systems all the effective exchange integral (Jab) values between two donors and between donor and counter anions were successfully evaluated. In addition the other effective parameters (tab, Ueff) were also obtained. Finally relationship between integral pair (J, J′) and transition temperature (TSC) was discussed. The two- and one-dimensional magnetic networks are essential for these crystals.

TMTSF salts with polycyano organic anions without Bechgaard structures

Eleven semiconducting TMTSF salts with 2-alkyl-1,1,3,3-tetracyanoallyl anion (R-TCA - ) derivatives are investigated. The 1:1 compositions are observed for six salts and those of the other salts are regarded to deviate from 1:1 on the basis of the observation of the optical absorption band around 4000 cm -1 , which indicates mixed valence states. Hydroxyethoxy-TCA - (HE-TCA - ) provides the first example of 4:3 salt based on TMTSF, which contains one-dimensional (1D) columnar structure similar to those of δ-type BEDT-TTF salts. HCTMM 2- (C[C(CN) 2 ] 3 2- ) salt consists of the layered structure composed of 1D stacks and a TMTSF molecule orthogonal to the others. Comparison of the bond length suggests charge separations in the 4:3 HE-TCA - and the HCTMM 2- salts.

Variable band filling in organic conductor with proton donor/acceptor H nTNBP (2− n)− (TNBP: 3,3′,5,5′-tetranitro-4,4′-biphenyldiol)

H 2TNBP (3,3′,5,5′-tetranitro-4,4′-biphenyldiol) is known as a proton donor forming monoanion HTNBP 1− and dianion TNBP 2−. We have studied radical salts of TTF-series with TNBP 2− as counter anion, and found two kinds of ET (BEDT-TTF) salts, namely 4:1 and 2:1. In the 2:1 salt, it is strongly suggested that the formula is represented as (ET 0.5+) 2(HTNBP 1−) by the optical data and relatively high conductivity. The chemical species of anion part is determined by the chemical shift of 13 C -CP/MAS NMR in the solid state. Coexistence of TNBP 2− and HTNBP 1− is observed in 3:1 BO salt and 2:1 TMTSF salt, though only TNBP 2− exists in 4:1 ET salt. The 2:1 TMTSF salt exhibited optical transitions at ca. 2500 and 7500 cm −1 simultaneously in the reflection spectrum, however, the intensity ratio differs from crystal to crystal. It suggests that the charge of TMTSF varies between +0.5 and +1.0, and the formula is represented as ( TMTSF (1−x/2)+) 2( TNBP 2−) 1−x( HTNBP 1−) x (0<x<1) .

The search for new superconductors at argonne national laboratory

The organic superconductors program at Argonne National Laboratory was started by Jack M. Williams in 1981. Initial studies centered around structural effects in the TMTSF salts. Since 1984, the major emphasis has been on salts of the electron donor bis(ethylenedithio)tetrathiafulvalene (ET). Important milestones include the discovery of a number of new superconductors, among them: β-(ET) 2 IBr 2 (T c =2.8 K) and β-(ET) 2 AuI 2 (T c =4−5 K). These salts led to the first structure-properties relationship in ET salts. κ-(ET) 2 Cu[N(CN) 2 ]Br (T c =11.8 K, ambient pressure) and κ-(ET) 2 Cu[N(CN) 2 ]Cl (T c =12.5 K at 0.3 kbar pressure) are still the cation radical superconductors with the highest known transition temperatures. The families of κ L - and κ H -(ET) 2 [M(CF 3 ) 4 ](solvent) salts (T c =2−11 K) yielded at least 25 new closely related organic superconductors with M=Cu, Ag, Au, and solvent=1,1,2-trihaloethane. β″-(ET) 2 SF 5 CH 2 CF 2 SO 3 (T c =5 K) was the first superconductor where both the radical cation and the charge balancing anion were organic. Recent results in the synthesis and characterization of ET salts with anions similar to SF 5 CH 2 CF 2 SO 3 − are presented.

Electrical properties of organic conductors at high temperature

Physical properties up to 600 K are studied for (TMTSF) 2 PF 6 , (TMTTF) 2 PF 6 , (DMET) 2 I 3 , (DMET) 2 AuI 2 , (DIMET) 2 I 3 , (DIMET) 2 AuI 2 and κ-(ET) 2 Cu(NCS) 2 . It has been found that all the compounds decompose between 450 and 500 K by TG-DTA measurement. Electric resistivity and thermopower of the TMTSF and DMET salts are metallic as were expected from their temperature dependence below 300 K. The TMTTF and DIMET salts show constant thermopower, though their resistivity is metallic. This suggests that the TMTTF and DIMET salts are intrinsically non-metallic even above room temperature. Rapid decrease in resistivity found for many of the Q1D salts before their decomposition suggests the partial escape of anions and change in the carrier concentration. κ-(BEDT-TTF) 2 Cu(NCS) 2 shows hysteretic temperature dependence of the resistivity above 300 K.

Quantized Hall Effect-like behavior in (DMET-TSeF) 2AuI 2

The Hall effect in the FISDW (Field Induced Spin Density Wave) states in the quasi one-dimensional (DMET-TSeF) 2 AuI 2 shows step like hehavior which is similar to the quantized Hall effect observed in the TMTSF salts. The result contrasts with that for the salt (DMET-TSeF) 2 AuCl 2 of the same family where the Hall voltage is not clearly quantized. The Hall voltage for the AuI 2 salt becomes 0 above 24 T which is higher than that observed in the AuCl 2 salt (18T). The absolute values of the Hall voltages at the corresponding phase indices are about one sixth of the values observed in the TMTSF system. Possible explanations are moving FISDWs and different pinning mechanisms peculiar to each salts in the DMET-TSeF family.

Control of electronic properties of organic conductors by uniaxial strain method

We succeeded to artificially control the electronic properties of some BEDT-TTF and TMTSF salts by shortening the intermolecular distance along a desired direction without changing those along others (uniaxial strain method). We were able to make α-(BEDT-TTF) 2 KHg(SCN) 4 and its NH 4 analogue behave similarly to each other by controlling the lattice parameters a and c independently. The former compound was found to show the superconductivity like the latter and the latter the nesting instability like the former. It is possible to systematically understand the electronic properties of these materials as a function of c/a. In (TMTSF) 2 PF 6 , contrary to conventional expectations, the reduction of the lattice parameter a, but not those perpendicular to a, plays a key role to suppress the SDW and induces the superconductivity.

Direct evidence of dimensionality enhancement of Q1D TMTSF and DMET salts

Magnetoresistance measurement was carried out for quasi-one-dimensional (Q1D) organic superconductors, (TMTSF) 2 PF 6 , (DMET) 2 AuI 2 and (DMET) 2 I 3 at 1.2 K up to about 15 kbar. The third angular effect (TAE) is observed in their metallic state as an hump anomaly of the angular dependence of the magnetoresistance. It has been found that the angular width of the TAE hump anomaly increases with increasing hydrostatic pressure. This is the first direct evidence of the dimensionality enhancement by pressure for the Q1D superconductors, where the dimensionality is defined as the anisotropy of the transfer integrals, t y /t x .

Charge and spin dynamics of TMTSF and TMTTF salts

A comprehensive investigation of the charge and spin dynamics of the quasi one-dimensional organic compounds (TMTSF) 2 X and (TMTTF) 2 X with X= PF 6 , AsF 6 , Br, and ClO 4 has been performed in the temperature range from 2 K up to 500 K. By substitution of the organic donors or inorganic anions, the dimensionality of the systems can be tuned. While the resisitivity strongly increases in (TMTTF) 2 X below 100 K. the susceptibility measured by electron spin resonance varies only slightly.

TTF based charge transfer salts of [Cr(NCS)4(phen)]−: bulk magnetic order and crystal structures of the TTF, TMTTF (tetramethyltetrathiafulvalene) and TMTSF (tetramethyltetraselenafulvalene) derivatives †

A series of new charge transfer salts of electron donors based on tetrathiafulvalene, TTF, with the anion [Cr(NCS)4(phen)]− (phen = 1,10-phenanthroline) have been prepared. Single crystal X-ray diffraction provided structures for salts with donors TTF, TMTTF, tetramethyltetrathiafulvalene, and TMTSF, tetramethyltetraselenafulvalene. The salts were found to be [TTF][Cr(NCS)4(phen)] at 293 K, [TMTTF][Cr(NCS)4(phen)]·CH2Cl2 at 250 K and [TMTSF]3[Cr(NCS)4(phen)]2·0.5CH2Cl2 at 293 K. For the TMTSF compound the solvent is disordered at 293 K so the structure was also solved at 120 K when the solvent becomes ordered. All of the salts have multiple S⋯S or S⋯Se close contacts between the anions and cations. The TTF salt is a bulk ferrimagnet with Tc = 9 K and is an insulator whereas the TMTTF salt is an antiferromagnetic (TN = 3.0 K) insulator. Both have close atomic interactions of the phen–donor π-stacking type, but the TMTTF salt also contains close anion–anion contacts and dimerised cations whereas the TTF salt consists of stacks of alternating cations and anions. The TMTSF salt is a paramagnetic semiconductor and does not display π stacking but has close inter-donor Se⋯Se contacts and isolated anions which afford the magnetic and transport properties.

Possible Spin-Singlet Superconductivity in (TMTSF)2X: Superconducting Transition Temperature in a Magnetic Field

We study the transition temperature T c ( H ) of a quasi-one-dimensional (Q1D) superconductivity which is derived from the quantum effect of electron motion in a strong magnetic field. We calculate T c ( H ) of both isotropic and anisotropic superconductivity by taking account of the optimal momentum of the Cooper pairs and the effect of higher harmonic terms along the second conducting axis in the tight-binding model. We find that, although T c ( H ) of the spin-singlet superconductivity is strongly suppressed by the Zeeman effect, the suppression of T c ( H ) is not very severe if we take the optimal pair-momentum and the higher harmonic terms into account. The obtained T c ( H ) for the spin-singlet superconductivity is consistent with the experimental results in TMTSF salts.

Molecular Conductors Based on the [Au(dmit)2] Complex Anion

The salts TTF 3 [Au(dmit) 2 ] 2 , TMTSF 3 [Au(dmit) 2 ] 2 and BEDT-TTF[Au(dmit) 2 ] have been prepared. The electrical conductivities of the salts are 0.1 Scm -1 , 6.5×10 -2 Scm -1 and 1.8×10 -4 Scm -1 respectively. The salts exhibited the temperature dependance of semiconductors, with band gaps of 60 meV, 30 meV and 500 meV for the TTF, TMTSF, and BEDT-TTF salts respectively. The salts are compared with their [Ni(dmit) 2 ] analogues

Design of Organic Superconductors: Successes and Disappointments

Abstract Some structural and chemical approaches applied to the construction of organic superconductors are discussed. A retrospective view from the first quasi one-dimensional superconductors (TMTSF salts) to quasi two-dimensional BEDT-TTF salts and recent three-dimensional fullerene complexes is presented.