ET Salt Research Articles

Mn‐Containing Paramagnetic Conductors with Bis(ethylenedithio)tetrathiafulvalene (BEDT‐TTF)

Two novel paramagnetic conductors have been prepared with the organic donor bis(ethylenedithio)tetrathiafulvalene (BEDT‐TTF = ET) and paramagnetic Mn‐containing metallic complexes: κ′‐ET4[KMnIII(C2O4)3]∙PhCN (1) and ET[MnIICl4]∙H2O (2). Compound 1 represents the first Mn‐containing ET salt of the large Day’s series of oxalato‐based molecular conductors and superconductors formulated as (ET)4[AM(C2O4)3]∙G (A+ = H3O+, NH4+, K+, ...; MIII = Fe, Cr, Al, Co, ...; G = PhCN, PhNO2, PhF, PhCl, PhBr, ...). It crystallizes in the orthorhombic pseudo‐κ phase where dimers of ET molecules are surrounded by six isolated ET molecules in the cationic layers. The anionic layers contain the well‐known hexagonal honey‐comb lattice with Mn(III) and H3O+ ions connected by C2O42- anions. Compound 2 is one of the very few examples of ET salts containing ET2+. It also presents alternating cationic‐anionic layers although the ET molecules lie parallel to the layers instead of the typical almost perpendicular orientation. Both salts are semiconductors with room temperature conductivities of ca. 2 x 10-5 and 8 x 10−5 S/cm and activation energies of 180 and 210 meV, respectively. The magnetic properties are dominated by the paramagnetic contributions of the high spin Mn(III) (S = 2) and Mn(II) (S = 5/2) ions.

Structural Characteristics in a Stable Metallic ET Salt with Unusually High Oxidation State (ET: Bis(ethylenedithio)tetrathiafulvalene)

Abstract Electrolysis of ET in a mixed solvent of methanol/CHCl3 with bromide yielded (ET)3(Br3)5, in which an unusually high formal charge of ET5/3+ is achieved. ET forms a two-dimensional conducting sheet incorporating Br3−. Despite the high formal charge of the ET cations, the strong electrostatic attractions between the ET cations and Br3− support aggregation of the ET cations, allowing sufficient π–π overlap for delocalization of the π-electrons. The crystal shows stable metallic behavior down to low temperatures, being consistent with the metallic band structure with two-dimensional Fermi surfaces and Pauli-like paramagnetism down to low temperatures. The lack of any indication of instability in the metallic phase due to correlation effects is considered to result from an ionic-crystal-like structure, in which the lattice is more resistant toward lattice deformation and/or shrinkage than in typical partially oxidized salts with low formal charges.

Charge fluctuation of the superconducting molecular crystals

In recent years, concern has been raised about the charge fluctuation of the superconducting transition in the loosely dimerized molecular conductors. Not only the observation of the charge fluctuation is of considerably important but also the understanding of the mechanism of the fluctuation. We have observed degree of charge fluctuation of several β″-type ET salts. The β″-type ET salt is one of the best model compounds because the direction of the largest inter-site Coulomb interaction is perpendicular to that of the largest transfer integral. This structural property allows us to examine the role of inter-site Coulomb interaction from the viewpoint of the inter-molecular distance. The difference in the molecular charges between the charge rich site and the charge poor sites, Δ ρ, is correlated with the conducting behavior; the superconducting materials have the small but finite Δ ρ, whereas Δ ρ of the insulating (metallic) materials is large (almost zero). After the analysis of the configuration in the inter-molecular distances, we have found that the degree of fluctuation, Δ ρ, is attributed to the number of the most stable charge distribution(s), N S, and the number of the energy levels of the allowed charge distribution, N A. The superconducting materials belong to the condition of N S≥2 and N A≥2. Indeed, this condition contributes to the fluctuation of the molecular charges.

Charge ordered state and frustration of site charges in(ET)5Te2I6and(BETS)5Te2I6

We have carried out x-ray structural analysis and have studied the temperature dependence of vibrational spectra and electrical resistivity under uniaxial strain for ${(\mathrm{ET})}_{5}{\mathrm{Te}}_{2}{\mathrm{I}}_{6}$ and ${(\mathrm{BETS})}_{5}{\mathrm{Te}}_{2}{\mathrm{I}}_{6}$ [$\mathrm{ET}=\mathrm{bis}(\text{ethylenedithio})\text{tetrathiafulvalene}$ and $\mathrm{BETS}=\mathrm{bis}(\text{ethylenedithio})\text{tetraselenafulvalene}$]. In the low-temperature insulating phase for each salt, a charge-sensitive mode ${\ensuremath{\nu}}_{2}$ exhibits peak splitting, and a vibronic ${\ensuremath{\nu}}_{3}$ mode shows factor group splitting. This observation confirms the charge ordered state. The distribution of site charges is determined from the factor group analysis of the vibronic ${\ensuremath{\nu}}_{3}$ mode, which is in agreement with that determined from x-ray structural analysis of the ET salt. In the high-temperature conducting phase, the vibronic ${\ensuremath{\nu}}_{3}$ mode is smeared out whereas the frequencies of the two charge-sensitive modes are almost unchanged over the whole temperature range. We have proposed a model wherein the highly conducting state is ascribed to the frustration between the site-charge distributions in the insulating state and the other distribution, which contributes to a reduction in the intersite Coulomb interaction along the stacking and diagonal directions, respectively. Our conjecture is supported by the temperature dependence of the electrical resistivity under the uniaxial strain.

BEDT-TTF Template Crystallization with Use of an Electron Donor Molecule with Dodecanethiol

Organic conductors and superconductors based on charge-transfer salts, such as (BEDT-TTF)2X [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene or ET], exhibit a wide variety of interesting phenomena that would be useful in numerous sensor and electronic applications. Unfortunately, there has been difficulty in creating thin, patterned films of these materials, which would be required for any potential application. In an effort to resolve this problem, a BEDT-TTF-like compound with a dodecanethiol chain has been synthesized and characterized. The application of this new compound in the templating of ET patterned films through the use of microcontact printing and dip-pen nanolithography is currently under investigation. In addition, an ET salt pattern has also been prepared through the use of a general hydrophilic surface derivatization method. Preliminary results will be discussed with particular attention paid to the success of pattern reproduction and identity of the templated species, as well as how thes...

Thin Films and Surface Patterning with BEDT-TTF Based Charge Transfer Salts

Densely covered (ET)X 2 thin films can be selectively electrodeposited on gold electrodes. The insulating (ET)X 2 films are converted to (ET) 2 X conductive films through a novel conproportionation reaction. Both ET and ET salt patterns can be prepared with the PDMS stamping technique with use of an ET derivative with a dodecanethiol chain for surface derivatization. These solution procedures open up the possibility to prepare conductive and superconductive charge-transfer salt thin films as well as patterns.

Physical properties of (ET)3(MnCl4)(TCE) and the related salts

The ET salt with a magnetic counter ion (ET) 3 (MnCl 4 )(TCE) (TCE = 1,1,2-trichloroethane) exhibits pressure-sensitive, complicated electrical behavior due to its semimetallic electronic structure. On the other hand, stronger d-n coupling is suggested in (ET) 3 (MnCl 3 ) 2 (C 2 H 5 OH) 2 , which has infinite magnetic chains with semimetallic conducting behavior.

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) .

Superconductivity of a θ-type et salt prepared using CuCN AND PH 4 PN(CN) 2 as electrolyte

A θ-type ET salt θ-(ET) 2 Cu 2 (CN)[N(CN) 2 ] 2 has been obtained by electrocystallization in PhCN, using Ph 4 N[N(CN) 2 ] and CuCN as electrolytes. Some of them showed anomalous behaviors for Low-Field-Microwave-Absorption (LFMA) measurements, which were one of the characteristics of superconductivity. However, the others did not show the LFMA signals, indicating that there are two phases in this compound having different electronic states at low temperature.

Novel sulfonate containing ET based synmetals

Electrocrystallization of ET in the presence of aromatic and aliphatic sulfonate anions has led to many new ET salts. These new ET based sulfonate complexes are characterized with use of x-ray diffraction, four-probe conductivity measurements, ESR and Raman spectroscopies. A new κ(4 × 4) donor packing motif was observed in (ET) 2(C 6H 5CH 2SO 3)(H 2O). Strong hydrogen bonding between the sulfonate anion S-O and the donor ethylene C-H provides the driving force for the ET salt formation. Many of these new sulfonate salts are highly conductive with some remaining metallic to ~ 4 K.

Correlation effects in ET compounds

The ET salt family is described by a two parameter 2D Fermi surface model, in which bandwidth and departure from perfect nesting can be varied. We have studied the effect of spin fluctuations on the normal state properties in a RPA approach. The calculated NMR relaxation rate exhibits a peak in 1/T 1T, which strongly decreases when the departure from perfect nesting of the Fermi surface and the bandwidth increase. These results are in perfect agreement with NMR experiments done in (BEDT-TTF) 2 X (X= Cu[N(CN) 2]Br, Cu(NCS) 2), at least qualitatively. We investigate the superconductivity pairing due to spin fluctuations and a model of quasi-2D electrons close to SDW instability, described by a two-parameter Fermi surface model, in which bandwidth and departure from perfect nesting can be varied. The coupling constants of various channels are calculated. They strongly depend on the nesting properties of the Fermi surface and on the bandwidth.

A new stable organic metal in the well known family of radical cation α-salts: α-(BETS)2TlHg(SeCN)4. Synthesis, structure and properties

The stable organic metalα-(BETS)2TlHg(SeCN)4 has been obtained by means of electrochemical oxidation of bis(ethylenedithio)tetraselenafulvalene (BETS) in 1,1,2-trichloroethane (TCE) with the addition of absolute ethanol in the presence of an electrolyte consisting of 18-crown-6, TlSeCN and Hg(SeCN)2 . The salt is isostructural with the ET-based salt with the same anion and ET and BETS salts with KHg(SCN)4 as well. The number of the side-by-side Se‥Se, Se‥S and S‥S contacts in each radical cation layer was twice that in comparison with the corresponding ET salt. Reflectivity spectra and magnetoresistance were studied. The Shubnikov–de Haas effect and angular oscillations of the magnetoresistance were observed. Optical and transport properties of α-(BETS)2TlHg(SeCN)4 were compared with those of the analogous ET salt.

Rational design of organic superconductors through the use of the large, discrete molecular anions M(CF 3) 4− ( M= Cu, Ag, Au) and SO 3CF 2CH 2SF 5−

A new approach to the synthesis of organic superconductors has recently been pioneered which involves the use of large, discrete, molecular anions as the charge-compensating entities in these charge transfer salts. The organic electron-donor molecule bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF or ET) has been electrocrystallized with the novel organometallic M(CF 3) 4 − ( M = Cu, Ag, and Au) anions in a variety of 1,1,2-trihaloethane solvents. Over twenty organic superconductors have been synthesized which can be described by the general formula (ET) 2 M(CF 3) 4(1,1,2-trihaloethane). These solvated salts are shown to have highly anisotropic physical properties which can be tuned via modifications of each of their three molecular components: ET electron donor molecule, M(CF 3) 4 − anion, and neutral 1,1,2-trihaloethane solvent molecule. Superconductivity has also been observed in an ET salt containing the discrete SF 5CH 2CF 2SO 3 − anion with an onset temperature near 5.2 K.

Design of ET salt with Cu(I) and Hg(II) containing anions

Abstract New salt (ET) 5 [CuHg(SCN) 4 ] 2 together with the superconductor (ET) 2 Cu(NCS) 2 in (Hg(SCN) 2 +CuNCS+NCS − ) system were synthesized. Their crystal structure and conductivity are discussed.

The first ET salt with a metal complex anion containing a selenocyanate ligand, (ET)2TIlHg(SeCN)4 : synthesis, structure and properties

Synthesis of the first ET salt with a metal complex anion containing a selenocyanate ligand, α-(ET) 2 TlHg (SeCN) 4 , and its structure are reported. The temperature dependence of conductivity and thermopower in various crystallographic directions and the Shubnikov-de Haase oscillations have been studied. Distinctions of the bond length distributions in three crystallographically independent ET have been found, which may result in some charge localization in the radical-cation layer. A detailed comparison of the structure and properties of the compound obtained to its thiocyanate analog has been performed. It has been shown that in the α-(ET) 2 TlHg(SeCN) 4 salt there is no low-temperature phase transition characteristic of the α-(ET) 2 MHg(SCN) 4 compounds with M=K, Tl and Rb