On the stoichiometry of the organic metal bis(ethylenedioxy)-tetrathiafulvalene chloride

We report on Shubnikov-de Haas and de Haas-van Alphen measurements of an organic metal based on the molecular donor bis(ethylenedioxy)-tetrathiavulvalene (= BEDO-TTF) and the anion Cl- with H2O molecules. The observed single oscillation frequency perfectly follows the two-dimensional dependence F = F-0/cos Theta with F-0 = (4890 +/- 20) T. The Fermi-surface area of similar to 50% of the first Brillouin zone proves a quarter-filled band, i.e., a 2:1 stoichiometry of the BEDO-TTF donor with respect to the anion. The apparent discrepancy to X-ray data which give a 1:1 ratio between BEDO-TTF and Cl- is understood by replacement of H2O molecules with (H3O)(+) ions. The proposed stoichiometry therefore is (BEDO-TTF)(2)(+) (H5O2)(+)(Cl-)(2). The cyclotron effective mass is m(c) (1.54 +/- 0.05)m(e)/cos Theta when deduced from the temperature dependence of the fundamental oscillation amplitude, but strongly reduced when extracted from higher harmonics. This and the strong harmonic content of the oscillations signals an influence of the two-dimensional electronic structure and can by qualitatively understood by applying the concept of magnetic interaction to the effect of the oscillatory chemical potential.