Synthesis, Structure and Physical Properties of a Two-Dimensional Organic Metal, Di[bis(ethylenedithiolo)tetrathiofulvalene] triiodide, (BEDT-TTF)⁺₂ I⁻₃

Abstract

Electrical oxidation of bis(ethylenedithiolo)tetrathiofulvalene (BEDT-TTF) 1 in a nitrogen saturated tetrahydrofurane solution containing (n-C₄H₁₀)₄N⁺I₃ as electrolyte results in the deposition of crystal plates of (BEDT-TTF)⁺ ₂I₃ (C₁₀H₈S₈)₂I₃. These two dimensional crystals are triclinic, space group P1, a = 9.211(2), b=10.850(4), c = 17.488(5) Å, α=96.95(2), β = 97.97(2), γ = 90.75(2)°, V = 1717 ų, M_r = 1150.01, Z=2, D_x =2.22 g cm⁻³. R_w = 0.037 for 2462 observed reflections. Sheets of alternating cations and anions occur in the compound. The most prominent intermolecular S-S contacts (∼3.5Å) are found between the stacks of BEDT-TTF. The stacking of the molecules and the interstack contacts give the compound a two-dimensional character which influences the physical properties.

The room temperature conductivities in the ab plane range typically between 60 and 250 (ω cm)⁻¹. The conductivity anisotropy in this plane is small with a ratio of σ_a/σ_b⩽ 2, while in the direction perpendicular to the ab-plane at least a 1000 times smaller value is found. The temperature dependent conductivity and thermopower measurements show that in the temperature range between 135 and 300 K the crystals 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 ε_f gives a value of ε_f ≈0.3 eV.