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 or ganic metal based on the molecular donor bis(ethylenedioxy)-tetrathiavulval ene (= BEDO-TTF) and the anion Cl<SUP>-</SUP> with H<INF>2</INF>O molecules . The observed single oscillation frequency perfectly follows the two-dimen sional dependence F = F<INF>0</INF>/cos Theta with F<INF>0</INF> = (4890 +/ - 20) T. The Fermi-surface area of similar to 50% of the first Brillouin zo ne 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 whi ch give a 1:1<SUP></SUP> ratio between BEDO-TTF and Cl<SUP>-</SUP> is under stood by replacement of H<INF>2</INF>O molecules with (H<INF>3</INF>O)<SUP> +</SUP> ions. The proposed stoichiometry therefore is (BEDO-TTF)<INF>2</INF ><SUP>+</SUP> (H<INF>5</INF>O<INF>2</INF>)<SUP>+</SUP>(Cl<SUP>-</SUP>)<INF> 2</INF>. The cyclotron effective mass is m<INF>c</INF> (1.54 +/- 0.05)m<INF >e</INF>/cos Theta when deduced from the temperature dependence of the fund amental oscillation amplitude, but strongly reduced when extracted from hig her harmonics. This and the strong harmonic content of the oscillations sig nals an influence of the two-dimensional electronic structure and can by qu alitatively understood by applying the concept of magnetic interaction to t he effect of the oscillatory chemical potential.