BEDT-TTF Layers Research Articles

Low temperature crystal structure of the organic metal ([2H8]BEDT–TTF)4Cl2·6D2O [BEDT–TTF=bis(ethylenedithio)tetrathiafulvalene

([2H8 ]BEDT–TTF)4Cl2 ·6D2O [BEDT–TTF=bis(ethylenedithio)tetrathiafulvalene] exhibits a transition from semimetal to semiconductor at T=160 K (Rosseinskyet al., J. Mater. Chem., 1993, 3, 801). This electronic transition is accompanied by a structural transition that is characterized by the reversible appearance of superstructure reflections corresponding to the doubling of the cell parameter b and a change in space group from Pcca to Pbcn. The crystal structures are very similar above and below T and the calculated intermolecular transfer integrals scarcely change. In contrast, from the difference in intramolecular bond lengths, it is clear that the two crystallographically independent BEDT–TTF molecules carry different charges at low temperature, suggesting that a degree of ionicity arising in the BEDT–TTF layer is responsible for the change in electrical behaviour.

NMR Study of (BEDT-TTF)(TCNQ) at selective nuclear sites

The charge transfer complex, (BEDT-TTF)(TCNQ), is a hybrid system which consists of one-dimensional (1D) columns and two-dimensional (2D) BEDT-TTF layers and is known to be a paramagnetic insulator below room temperature. In order to examine the electronic structures of 1D and 2D layers separately, we prepared the complex between TCNQ and 13C,deuterium-substituted BEDT-TTF. We report the site-selective NMR study on this sample and discuss the electronic structure of the 1D and 2D electronic subsystems.

Electronic structure of the organic conductor α-(BEDT-TTF)2I3sstudied by angle-resolvedand core-level photoelectron spectroscopy

The electronic structure of the organic conductor \ensuremath{\alpha}-(BEDT-TTF${)}_{2}$ ${\mathrm{I}}_{3}$ has been studied by photoelectron spectroscopy on in situ cleaved crystals. The valence-band regime consisted of eight distinct features with binding energies between 1.2 and 10.9 eV. Angle-resolved measurements showed no dispersion along the \ensuremath{\Gamma}-Z (${\mathrm{k}}_{\mathrm{z}}$ ) direction, i.e., perpendicular to the conducting BEDT-TTF layers, for the structures in the valence-band regime. Angle-resolved measurements along the a and b axis, i.e., approximately along a and b, gave no evidence that the structures closest to the Fermi level show any dispersion in the highly conducting plane. The present experiment sets an upper limit for the dispersion of about 0.25 eV. The lack of any (observable) dispersion and the lack of a sharp Fermi edge in spite of \ensuremath{\alpha}-(BEDT-TTF${)}_{2}$ ${\mathrm{I}}_{3}$ showing metallic transport properties are thoroughly discussed. It is argued that correlation effects or the calculated narrow-gap semiconductor band structure of \ensuremath{\alpha}-(BEDT-TTF${)}_{2}$ ${\mathrm{I}}_{3}$ are the most probable explanations for the latter phenomena. From the photon energy dependence, the atomic origin of several structures in the valence-band regime is suggested. Core-level spectroscopy on the I 4d level revealed the presence of one bulk component and a surface shifted component. S 2p core-level spectra revealed no surface or chemically shifted components. The large width of the bulk components of these core levels is primarily ascribed to phonon broadening.

Three-Component Organic Conductors; (BEDT-TTF)2MM′(SCN)4

Abstract New organic conductors which contain a magnetic cation of Co2+ (d7, S=3/2), (BEDT-TTF)2MCo(SCN)4 [M=K, Rb, Cs], and related materials were prepared. The crystal structure analyses clarify that α-(BEDT-TTF)2MM′(SCN)4 [M=Rb, Cs, M′=Co, Zn] are isostructural and that these donor arrangements are α-type (strictly speaking θ-type). Although the band structure calculation affords the closed Fermi surface due to the large transverse overlap integrals, a metal-insulator (M-I) transition occurs at 20 K for α-(BEDT-TTF)2CsM′(SCN)4 [M′=Co, Zn] and at 190 K for α-(BEDT-TTF)2RbCo(SCN)4, respectively. The M-I transition temperatures of the former M=Cs salts are elevated by applying pressure. The magnetic susceptibilities of the M′=Co salts obey the Curie-Weiss law with θ=3–6K from 300 K to 20K. The relatively large θ might be originated from the superexchange of an anion network like… Co2+… NCS− as well as interaction through itinerant electrons in BEDT-TTF layer. Below 20 K the anomaly of χT was observed, whi...

Critical fields and mixed-state properties of the layered organic superconductor kappa -(BEDT-TTF)2I3.

We report on a systematic study of the thermodynamic superconducting properties on high-quality crystals of the quasi-two-dimensional (quasi-2D) organic superconductor $\ensuremath{\kappa}$-${(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}\ensuremath{-}\mathrm{T}\mathrm{T}\mathrm{F})}_{2}$${\mathrm{I}}_{3}$. Low-field magnetization measurements served to determine the lower critical field ${B}_{c1}$. Just below ${T}_{c}=3.5$ K, down to \ensuremath{\sim} 3 K, ${B}_{c1}$ perpendicular (\ensuremath{\perp}) to the highly conducting BEDT-TTF layers is found to be strongly suppressed below the sensitivity limit of the instrumental resolution. This might be an indication for single-vortex fluctuations expected for a strongly 2D superconductor. The critical field ${B}_{c2}^{*}$ determined by ac-susceptibility measurements is found to be related to an irreversibility line. The temperature and frequency dependences of ${B}_{c2}^{*}$ are consistent with well-established flux-creep models. The upper critical field ${B}_{c2\ensuremath{\perp}}$ extracted from magnetization measurements is consistent with specific-heat data. For both principal field orientations a crossover of the critical-field slopes $|\frac{d{B}_{c2}}{\mathrm{dT}}|$ towards a shallow tail close to ${T}_{c}$ is observed. Apparent discrepancies of the thermodynamic critical fields for parallel and perpendicular field orientation are found, indicating either a strongly reduced ${B}_{c1\ensuremath{\parallel}}$ or ${B}_{c2\ensuremath{\parallel}}$.

Three BEDT-TTF Salts

The crystal structures of 3,4;3',4'-bis(ethylenedithio)-2,2',5,5'-tetrathiafulvalenium hexafluorophosphate chloroacetyl chloride solvate, (1), C 10 H 8 S 8 1/2+ .1/2PF 6 - 1/2C 2 H 2 Cl 2 O, 3,4;3',4'-bis(ethylenedithio)-2,2',5,5'-tetrathiafulvalenium hexafluorophosphate tetrahydrofuran solvate, (2), C 10 H 8 Si 8 + .PF 6 - .1/2C 4 H 8 O, and 3,4;3',4'-bis-(ethylenedithio)-2,2',5,5'-tetrathiafulvalenium perchlorate, (3), C 10 H 8 S 8 + .ClO 4 - , have been determined. Compounds (1) and (2) crystallize in the P1 space group. The structure of (1) consists of layers of BEDT-TTF ions separated by planes of PF 6 - ions and chloroacetyl chloride molecules. The BEDT-TTF ions are stacked in a face-to-face manner with short S...S contacts linking these stacks. Compound (2) consists of layers of BEDT-TTF face-to-face dimers with PF 6 - ions and tetrahydrofuran (THF) molecules filling cavities separating these layers. As in compound (1), there are short S...S contacts between dimers resulting from the side-by-side mode of packing. Compound (3) crystallizes in the C2/m space group and consists of layers of BEDT-TTF ions, stacked in a staggered side-by-side manner with one type of S...S contact, separated by layers of perchlorate ions.

Synthesis, Structure and Physical Properties of the Molecular Magnetic Semiconductor (BEDT-TTF)4KFe(C2O4)3.C6H5CN(BEDT-TTF═BIS(ethylenedithio) Tetrathiafulvalene)

Abstract We report the synthesis and physical properties of compounds of BEDT-TTF with transition metal tris-oxalate anions. The crystal structure of one such compound, (BEDT- T TF)4KFe(C2O4)3C6H5CN (1) has been determined. The structure consists of alternating layers of BEDT-TTF and a hexagonal network of K+ and Fe(C2O4)3 3-. This material is a semiconductor with an activation energy of 0.14eV. Magnetic data obey the Curie-Weiss law with C = 4.44 (S=5/2) and O= −0.25K.

Electronic structure of alpha t-(BEDT-TTF)2I3: A photoemission study.

Photoemission experiments have been performed on in situ cleaved crystals of the organic superconductor ${\mathrm{\ensuremath{\alpha}}}_{\mathit{t}}$-(BEDT-TTF${)}_{2}$${\mathrm{I}}_{3}$. Nine distinct features are observed in the valence-band regime with binding energies between 1.4 and 11.2 eV. None of the valence-band structures shows dispersion along the \ensuremath{\Gamma}-Z (${\mathit{k}}_{\mathit{z}}$) direction, i.e., perpendicular to the conducting BEDT-TTF layers. The spectral intensity is zero at, and to within about 0.5 eV below the Fermi level. This is ascribed to the presence of a fairly large band gap in this direction, in agreement with band-structure calculations. The photoemission spectrum of the valence band shows remarkable changes when the photon energy is changed. These changes are qualitatively understood as a cross-section dependence. The observed photon energy dependence suggests that both p orbitals and the lower-lying s orbitals of the atoms constituting the BEDT-TTF molecule contribute significantly to the molecular orbitals forming the valence band. Core-level spectroscopy on the I 4d and S 2p levels revealed no surface or chemically shifted components. The large width of these core levels is ascribed primarily to phonon broadening, although a contribution from disorder cannot be completely ruled out.

Calculations of positron states in BEDT-TTF based organic superconductors

Summary form only given. Positron annihilation techniques (PAT) have been successfully applied in Fermiology and defect spectroscopy. Precise understanding of positron states is essential to interpret experimental results in the both fields, especially on complex materials as the current ones. BEDT-TTF based organic superconductors have two-dimensional alternating structures of anion and conducting BEDT-TTF layers. For Fermiology by PAT, higher positron annihilation rate with electrons on conducting BEDT-TTF layers is faborable. In this work, we have calculated positron distributions on a series of BEDT-TTF based organic superconductors, whose anion species are I/sub 3/, Cu(NCS)/sub 2/, Cu(CN)[N(CN)/sub 2/], Cu[N(CN)/sub 2/]Br and KHg(SCN)/sub 4/. The results will be discussed in connection with Fermiology by PAT.

Far-infrared reflectance of β″-(BEDT-TTF)2AuBr2: co-existence of free carriers and a single-particle gap at 2Δ=130cm−1

We have measured the temperature dependence of far-infrared reflectance of β ″-(BEDT-TTF) 2 AuBr 2 to investigate the possibility of SDW transition. The polarized reflectance in two-dimensional BEDT-TTF layers shows a highly anisotropic feature of conduction band. Both the E ⊥ c and the E ∥ c reflectance line-shapes, which are Drude-like at room temperature, demonstrate a clear dip below 100 cm −1 even at 40 K. The frequency-dependent conductivity at 7 K indicates the presence of a single-particle gap at 2 Δ =130 cm −1 assignable to the SDW nesting along c . The nesting fluctuation exists already at room temperature because of a strongly one-dimensional nature of the band dispersion.

Crystal and electronic structures and electrical, magnetic, and optical properties of two copper tetrahalide salts of bis(ethylenedithio)-tetrathiafulvalene.

We report the crystal and electronic band structures at 295 K and measurements of temperature-dependent magnetic susceptibility, electron paramagnetic resonance, optical reflectivity, conductivity, and thermopower for two copper tetrahalide salts of bis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF), (BEDT-TTF${)}_{3}$${\mathrm{CuBr}}_{4}$ and (BEDT-TTF${)}_{3}$${\mathrm{CuCl}}_{2}$${\mathrm{Br}}_{2}$. The two salts are isostructural with layers of BEDT-TTF having charges of 0 and +e separated by layers of pseudo-square-planar tetrahalides of copper (II). At ambient pressure these salts show conductivities near 1 S ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, and the magnetic properties indicate coupled localized spins present on both BEDT-TTF and the ${\mathit{d}}^{9}$ copper layers. At 60 K, there is a discontinuous drop in susceptibility, a sharpening of the electron paramagnetic resonance linewidth, and an increase in g value that we attribute to the loss of the contribution from the BEDT-TTF sheets. This may be associated with a Jahn-Teller distortion of the planar copper complexes. Below the transition temperature the susceptibility can be fitted to a quadratic layer antiferromagnet with J=15 K (${\mathrm{CuBr}}_{4}$) and 8 K (${\mathrm{CuCl}}_{2}$${\mathrm{Br}}_{2}$) and one spin per formula unit. Under pressure there is a very rapid increase in conductivity, to 500 S/cm at 24 kbar, the largest increase of conductivity under pressure in any molecular solid yet studied. There is a sharp transition from metal to insulator at temperatures rising to 111 K near 5 kbar, and falling to 80 K at 20 kbar. We consider that these salts are nearly metallic at ambient pressure, with strongly enhanced susceptibilities, but are brought to a fully metallic state under pressure as a result of increased intermolecular contacts.

Point contact spectroscopy of the electron-phonon interaction in the layered conductor beta -(bis(ethylenedithio)tetrathiofulvalene)2I3

A theory of point contact (PC) spectroscopy of the electron-phonon interaction in layered conductors with a quasi-two-dimensional energy spectrum of charge carriers is developed. The predicted anisotropy of the PC spectra is observed on heterocontacts of Cu and the organic metal beta -(BEDT-TTF)2I3. Two types of PC spectrum are revealed, which correspond to different orientations of the contact axis. The spectra measured on contacts with the current flow in the plane of the organic molecule layer display features caused by the interaction between the electrons and intramolecular vibrations. The spectra of the samples with the axis perpendicular to the BEDT-TTF layers have maxima reflecting the predominating contribution of the I3- anions in the processes of energy relaxation of the current carriers in such PCS.

Scanning tunneling and atomic microscopy images of organic salt conductor (BEDT-TTF)2TlHg(SCN)4

Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) images were obtained for the organic conducting salt (BEDT-TTF)[sub 2]TlHg(SCN)[sub 4], in which layers of the cations BEDT-TTF[sup 0.5+] alternate with layers of the anions TlHgSCN[sub 4][sup 2]. These images were interpreted on the basis of the partial and total electron density plots of a cation layer calculated by the extended Hueckel tight-binding (EHTB) electronic band structure method. The observed STM and AFM patterns are dominated by the ethylene group hydrogen atoms of the cations. The patterns and intensity of the STM images vary as a function of the gap resistance. A probable cause for this observation was discussed from the viewpoint of a conformational change in the ethylene groups of BEDT-TTF. Our work provides a strong support for the use of partial and total electron density plots in the interpretation of STM and AFM images. 24 refs., 7 figs.

Crystal structures and properties of two modifications of the molecular charge-transfer salt (BEDT-TTF)2Ag(CN)2[BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene

Crystal structures are reported of two modifications of the molecular charge-transfer salt (BEDT-TTF)2Ag(CN)2[BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene]. Both structures contain alternating layers of BEDT-TTF and [Ag(CN)2]–. Structure I is orthorhombic (P21212), a= 9.519(2)Å, b= 4.952(1)Å, c= 33.927(7)Å, Z= 2, R= 0.054, and structure II is monoclinic (P21/n), a= 10.996(1)Å, b= 4.281(1)Å, c= 34.093(5)Å, β= 93.13(2)°, Z= 2, R= 0.084. The modes of packing of the BEDT-TTF within layers are similar; the BEDT-TTF are inclined to the stacking axis and there are no significant S⋯S contacts within stacks. In adjacent stacks in I the molecules are inclined in the opposite sense, with dihedral angles of 77.3° and 81.5° for the two crystallographically independent layers, giving a number of short S⋯S distances. In II the dihedral angle between adjacent stacks is smaller (55.3°). Phase I is metallic from 300 to 1 K, while for II electron paramagnetic resonance (EPR) gives evidence for a metal–insulator transition near 100 K. The EPR linewidths are 25–30G (I) and 17–27 G (II). Analysis of the reflectivity data of I gives plasma frequencies of 9200 cm–1(‖a) and 7000 cm–1(‖b). The band structures of both materials are broadly similar, with a Fermi surface consisting of a nearly symmetric hole pocket centred at the zone centre of ca. 50% of the Brillouin zone. In I the two layers are found to have slightly different Fermi surfaces.

Imaging of an organic superconductor (BEDT-TTF)2(NH4)Hg(SCN)4 surface structure by scanning tunneling microscopy

The STM image of the two-dimensional organic superconductor (BEDT-TTF)2 (NH4)Hg(SCN)4 [BEDT-TTF: bis(ethylenedithio)tetrathiafulvalene] crystal showed a periodic structure, which is in good agreement with the molecular arrangement with the conductive BEDT-TTF layer in the crystal. The influence of the top anion layer on the STM image was observed, as was previously shown for the (BEDT-TTF)2KHg(SCN)4 crystal. The high stability of this surface has also been revealed.

Crystal structure and physical properties of a metallic charge-transfer salt: (BEDT-TTF)2Ag(CN)2.H2O

A new organic salt, bis[3,4;3',4'-bis(ethylenedithio)2,2',5,5'-tetrathiafulvalenium] dicyanoargentate(I) monohydrate [(BEDT-TTF)2Ag(CN)2.H20] (I), has been synthesized and its crystal structure and physical properties determined. The structure of (I) is different from those of the superconducting (fl), metallic (fl) and insulating (fl' and a') phases of anhydrous (BEDT-TTF)2X, where X is a triatomic monoanion, but is similar to the newly found superconducting salts x-(BEDT-TTF)213 and (BEDT-TTF)2Cu(NCS)2. Crystal data for (I): [C10HsS8]2[Ag(CN)2].H20, Mr=9472, monoclinic, P21, a = 12.593 (3), b = 8.642 (1), c = 16.080 (3) A, fl =109.33(2) ° , U =1651.2(5) A 3, Z=2, Dx= 1.905 g cm -3, Mo Ka X-radiation (graphite monochromator), ,~ = 0.71069 A, # = 16.03 cm-~, F(000) = 950, T= 298 K, R (wR) = 0-033 (0.034) for 3884 unique observed [F> 2o-(F)] data. The anion array consists of linear chains of three-coordinate Ag atoms linked by H20 to form layers parallel to the BEDT-TTF layers, with which they form. alternating stacks parallel to c. (I) is metallic over the temperature range 150-300 K. The EPR spectrum at 295 K consists of a single line at g = 2.007 (3), with a peak-to-peak width Hpp of 70 G (0.007 T). Between 50 and 300 K an activated spin susceptibility with Ea = 0.01 eV is observed.

Surface study of an organic superconductor k-(BEDT-TTF)2Cu(NCS)2 using a scanning tunneling microscope.

The surface structure of κ- (BEDT-TTF) 2Cu (NCS) 2 [BEDT-TTF, his (ethylenedithio) tetrathia-fulvalene] crystal was studied by scanning tunneling microscopy (STM). The STM image over the crystal be plane has revealed the individual BEDT-TTF molecules, whose arrangement is in good agreement with the projected image of the bulk structure determined by X-ray diffraction. The alternative stacked structure of BEDT-TTF layer and Cu (NCS) 2 layer was also revealed by the measurement over the ab-plane.

Origin of the anomaly in the d.c. conductivity in (BEDT-TTF) xTCNQ y salt

The electronic properties of monocrystalline BEDT-TTF) x TCNQ y salt have been investigated by measuring the d.c. conductivity, spin susceptibility and radiospectroscopic g factor and linewidth for various crystal orientations, from 370 K to 130 K. It is suggessted that the behaviour of the compound is determined both by the BEDT-TTF layers and TCNQ chains.