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IR absorption spectra from various K-phases of BEDT·TfF radical cation salts as well as from a-(BEDT·TIFhI, arc presented. From tbese spectra one special mode l'n 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 a-(BEDT·TrF)2I, at 135 K with respect to this vibrational model lin gives strong indication of a charge localization in at least one of the two crystallographic different stacks. Furthennore, we correlate the frequencies of the Cfh stretching modes of a variety of BEDT·TIF radical salts witb tbe superconducting phase transition temperatures. These frequencies are evidently not charge dependent, but significanUy reOect the strength of the interaction of the donor molecule with the respecti'Ye anion. It could be shown that phases with a higher Tc of their superconducting transition exhibit a smaller red shift of these frequencies, i.e., a less attractive donor-anion interaction.

The Fermi surface (FS) of the new organic metal (BEDT-TTF)4[Ni(dto)2] has been investigated by de Haas-van Alphen (dHvA) and Shubnikov-de Haas (SdH) experiments. In both quantum oscillations with two different frequencies Fα = 634 T and F β = 4245 T are observed. These results confirm the calculated FS, which consists of one-dimensional and twodimensional parts separated by a small energy gap. It is shown that the temperature and field dependence of the oscillation amplitudes can be well described by standard Lifshitz-Kosevich theory considering the magnetic breakdown by the so-called coupled network model. Beatings of the oscillation amplitudes are observed by field-dependent dHvA and SdH experiments. They can be explained by a slightly warped FS showing that this material represents a quasitwo-dimensional electronic system. The observed beating nodes in the dHvA and SdH signals appear at different magnetic fields. This fact is discussed in terms of additional scattering mechanisms.

The structure and the temperature dependence of the resistivity, thermopower and ac-susceptibility of the new organic metal (BEDO-TTF)2ReO4(H20) was investigated. The resistivity and therrnopower data indicate phase transitions at 213K, around 90K and 35K. Below 2.5K an onset to superconductivity is observed in the resistivity data. Superconductivity was suppressed in the resistivity at 1.3K by applying a magnetic field of about 0.2T. Ac-susceptibility data indicate that superconductivity is a bulk effect in (BEDO-TTF)2ReO4(H20) but the onset for superconductivity observed in the ac -susceptibility is only at 0.9K and the transition seems to be complete only at temperatures below 50mK. This broad transition might be due to some disorder in the structure created by the low temperature phase transitions.

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