Electrical oxidation of bis(ethylenedithiolo)tetrathiofulvalene (BEDT-TTF) 1 in a nitrogen saturated tetrahydrofurane solution containing (n-C4HIO)4N+I:i' as electrolyte results in the deposition of crystal plates of (BEpT-TTF)rI3' (CIOHgSshI3' These two dimensional crystals are triclinic, space group PI, a = 9.211(2), b = 1O.S50 ( 4) behave like a two dimensional metal. while below 135 K a metal-insolator transition occurs. The thermopower values indicate that the high temperature conductivity is dominated by holes. An estimate of the Fermi energy (F gives a value of (F '" 0.3 eV.

We report static magnetic susceptibility data, g(T), for the a and P phases ef the organic conductor di[bis(ethylenediothiolo)tetrathiafulvalene] tri-iodide [(BEDT-TTF)zlz] both at ambient pressure and under applied pressures of up to 10 kbar. Ambient pressure ESR data for both phases belo3 00 K are also presented. The temperature dependence of the spin susceptibility g~(T) for the superconductor P-(BEDT-i i'F)zlz is unusually small for organic conductors; the pressure derivative 81~~/3P is found to be only moderate in magnitude ( -3.3' per kbar) and independent of temperature beloved 300 K. For the a phase, Bing, /BP= -2.0%/kbar at ambient temperature.

Temperature -dependent thermopower-, dc-and microwave conductivity m~asurements on the two-dimensional organic metal (BEDT -TTF)2J; are reported.

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.

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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.

Abstract. The electronic properties of the organic superconductor (BEDO-TTF)2 ReO4.(H20 ) were investigated by temperature dependent resistivity, ESR, Hall effect and magnetoresistance measurements. Shubnikov-de Haas (SdH) oscillations were observed in magnetic fields up to 24 T in the temperature range 0.5 K to 4.2 K. The electronic band structure of (BEDO-TTF)2 ReO 4. (H20) was calculated by employing the extended Hiickel tight binding method on the basis of its room temperature crystal structure. The two observed SdH frequencies of 75 T and 37 T correspond very well with two cross-sectional areas of the hole and electron Fermi surface pockets obtained from the tight binding calculation. From the temperature dependence of the SdH oscillation amplitudes, the cyclotron effective mass (me) belonging to the larger and smaller pockets were found to be 0.9 m o and mc = 1.15 m 0 respectively. Measurements of the angular dependence of the SdH frequencies show no deviation from that expected for a cylindrical Fermi surface. In terms of our tight binding calculations and experimental measurements, probable causes for the 213K and 35 K phase transitions are discussed. The calculations show that (BEDO-TTF)2 ReO 4.(H20 ) is a two dimensional semimetal but possesses a hidden nesting. The latter is likely to cause an SDW instability leading to the 35 K transition. The resistivity drop associated with the 213 K transition is likely to be induced by an abrupt increase in the relaxation time. The excellent agreement between the calculated and experimentally observed Fermi surface implies that, with decreasing temperature be-

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