J. Moldenhauer

FT-IR absorption spectroscopy of BEDT-TTF radical salts: charge transfer and donor-anion interaction

Abstract IR absorption spectra from various κ-phases of BEDT-TTF radical cation salts as well as from α-(BEDT-TTF) 2 I 3 are presented. From these spectra one special mode ν 27 representing the asymmetric ring C=C stretching vibration is assigned and the dependence of its frequency from the average charge on the donor molecule is given. The investigation of the metal-insulator phase transition of α-(BEDT-TTF) 2 I 3 at 135 K with respect to this vibrational model ν 27 gives strong indication of a charge localization in at least one of the two crystallographic different stacks. Furthermore, we correlate the frequencies of the CH 2 stretching modes of a variety of BEDT-TTF radical salts with the superconducting phase transition temperatures. These frequencies are evidently not charge dependent, but significantly reflect the strength of the interaction of the donor molecule with the respective anion. It could be shown that phases with a higher T c of their superconducting transition exhibit a smaller red shift of these frequencies, i.e., a less attractive donor-anion interaction.

Assignment of fundamental vibrations and estimation of electron-molecular vibration coupling constants for bis(ethylenedioxy)tetrathiafulvalene (BEDO)

Abstract The i.r. absorption and Raman spectra of BEDO are investigated. All fundamental vibrations are assigned using correlations between the obtained data and those for a well-known compound BEDT-TTF (ET). Comparative normal coordinate analysis has been done. The results are used for calculation of electron-intramolecular vibration coupling constants of BEDO based superconducting charge-transfer complexes. Comparison of these values with the constants obtained earlier for ET shows that BEDO is a perspective donor for searching new superconductors on its basis.

Thin films of the organic superconductor α1-(BEDT-TTF)2I3

Abstract α-(BEDT-TTF) 2 I 3 is a quasi-two-dimensional organic metal with a metal-insulator phase transition at 135 K. Thermal treatment at about 80° C leads to the metallic system α t -(BEDT-TTF) 2 I 3 , which becomes superconducting below 8 K. Thin films of the α-phase (between 500 and 3000 A thick) have been evaporated in high vacuum onto several substrates and characterised by means of X-ray diffraction, scanning electron microscopy (SEM), atomic force microscopy (AFM) and a low field microwave absorption technique. Depending on the temperature of the substrate and the evaporation rate, the films exhibit different degrees of microcristallinity, which under certain conditions can be strongly reduced and a completely covering film can be obtained. X-ray diffraction spectra reveal a high orientation with the c-axis perpendicular to the substrate and as well the successful conversion into the α t -phase by tempering. SEM and AFM investigations prove that the conversion takes place without reducing the mechanical quality of the films. Low-field microwave-absorption experiments show that the α t -films become superconducting with an onset at 9 K.

Superconductivity in polycrystalline pressed pellets of k-(BEDT-TTF)2I3

Abstract Investigations of the transport properties of single crystals and polycrystalline pressed samples of k -(BEDT-TTF) 2 I 3 are presented. In contrast to polycrystalline pressed samples of k -(BEDT-TTF) 2 Cu(NCS) 2 and k -(BEDT-TTF) 2 Cu[N(CN) 2 ]Br those of k -(BEDT-TTF) 2 I 3 show bulk superconductivity indicating that the loss of superconductivity in the first two types of samples is not due to the special structure of the k -phase, but probably due to a disproportionation of Cu 1+ under the anisotropic pressure during the preparation of the samples. Annealing the single crystals as well as the polycrystalline pressed samples of k -(BEDT-TTF) 2 I 3 at 75°C for a few days results in a structural transformation with an increase in the transition temperature for superconductivity (about 8K for the crystals). IR-spectra indicate a transformation into a structure which is identical with α t -(BEDT-TTF) 2 I 3 , which seems to be the thermodynamically most stable phase of the several (BEDT-TTF) 2 I 3 phases.

FT-IR investigations of BEDO-TTF and radical salts of BEDO-TTF

Abstract We investigated powder absorption spectra of the donor BEDO-TTF and some radical salts of BEDO-TTF, which are metals even at low temperatures. After an assignment of the bands in the neutral donor we found in the radical salts different vibrational frequencies of bands correlated with CO vibrations. These frequencies are directly related to the average charge on the donor molecule. Additionally frequencies of CH 2 -stretching vibrations exhibit differences, which can be ascribed to a varying strength of donor-anion interaction depending on the respective anion.

Polarized IR reflectance studies of the organic superconductor (BEDO-TTF)2ReO4(H2O)

Abstract Polarized reflectance spectra of single crystals of (BEDO-TTF)2ReO4(H2O) (BEDO-TTF, bis(ethylenedioxy)tetrathiafulvalene were measured at room temperature over the spectral range 500–8300 cm−1. The incident radiation was polarized parallel and perpendicular to the stacking axis of the donor molecules. By fitting a Drude model to the dispersion of reflectance the Drude parameters were obtained and, starting from the plasma frequency, the bandwidth 4t of the electronic system was derived. The real part of the dielectric function and the optical conductivity were determined by a Kramers-Kronig transformation. Both the optical conductivity and the bandwidth 4t are compared with values obtained by a band-structure calculation. In the frequency region below 2000 cm−1 the origin of the vibrational transitions is discussed and compared to transmission measurements of optical density.

Temperature-dependent IR reflectance investigations of the organic superconductor (BEDO-TTF)2 ReO4(H2O)

Abstract Polarized reflectivity spectra of single crystals of (BEDO-TTF) 2 ReO 4 (H 2 O) (BEDO-TTF = bis(ethylenedioxy)tetrathiafulvalene) were measured in the spectral range from 500 to 8500 cm −1 at six temperatures between 300 and 20 K. The polarization of the incident radiation was arranged parallel and perpendicular to the stacking axis of the donor molecules. The temperaturedependent reflectivity and the optical conductivity are presented. Changes at the metal-metal phase transition at 213 K and the metalsemiconductor phase transition at 35 K are discussed. The Drude parameters of the conduction electrons were obtained by fitting the reflectance data with a DrudeLorentz model, indicating that both phase transitions are accompanied by an abrupt change of the relaxation constant of the free charge carriers. Both phase transitions have an effect on the vibrational spectra and possible changes of the crystal structure at the phase transitions are discussed.

Thin Films of BEDT-TTF and BEDO-TTF Radical Cation Salts

Abstract In this communication we describe the fabrication and characterization of thin films of the radical cation salts α-, αt-(BEDT-TTF)2I3 and (BEDO-TTF)2.4I3. Thin films of these compounds were made by evaporation of the salts in high vacuum onto several substrates. They were characterized by X-ray diffraction, scanning electron microscopy, infrared absorption, Raman scattering, de-conductivity measurements and magnetically modulated microwave absorption. Depending on the temperature of the substrate and the evaporation rate, the films of α-(BEDT-TTF)2I3 exhibit different degrees of microcrystallinity which under the right conditions can be strongly reduced to obtain relatively smooth, completely covering films. Additionally such films have a high degree of orientation with respect to the substrate plane. The conversion into films of the superconducting αt-phase is possible and the high orientation is maintained. Due to grain boundaries, the dcconductivity is thermally activated, but the single grain...

Thin films of the organic conductor (BEDO-TTF)2.4I3

We report results on the fabrication of thin films of the organic radical cation salt (BEDO-TTF)2.4I3. The title compound is an organic metal with a room temperature dc-conductivity of 180 – 360 S/cm. On amorphous substrates as quartz glass and silicon wafers (SiSiO2), we found one or more orientations of the crystalline material with respect to the substrate plane. On NaCl there is only a single, very strong Bragg reflection, which is due to the very good, substrate-induced orientation of the film. Scanning electron microscopy reveals growth of grains on the quartz glass substrates, whereas on NaCl large smooth areas with 1 to 10 μm edge size are observed. The dc-conductivity shows a slightly activated behaviour with an activation energy of 20 meV and a room temperature value of 1 S/cm.

FT-IR investigations of (BEDT-TTF)2I3 radical salts

Abstract Fourier transform infrared absorption spectra of structurally different phases of the radical cation salt (BEDT-TTF) 2 I 3 in the range of the molecular vibrations of the donor are presented. The low-temperature spectra provide exact fingerprints of the different phases as long they have different unit cells. Additional differences concerning the positions of ethylene groups reflect themselves in the CH 2 -stretching vibrations. As a result, it could be shown that the ethylene groups of the α t -phase and β-phase must have a different order. Concerning the tempering of the different phases,it is shown that at least α t - and χ t - (BEDT-TTF) 2 I 3 are the same.