Charge-Ordering and Structural Transition in the New Organic Conductor δ'-(BEDT-TTF)₂CF₃CF₂SO₃

We report structural, transport, and optical properties and electronic structure calculations of the δ'-(BEDT-TTF)₂CF₃CF₂SO₃ (BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene) organic conductor that has been synthesized by electrocrystallization. Electronic structure calculations demonstrate the quasi-one-dimensional Fermi surfaces of the compound, while the optical spectra are characteristic for a dimer-Mott insulator. The single-crystal X-ray diffraction measurements reveal the structural phase transition at 200 K from the ambient-temperature monoclinic P2₁/m phase to the low-temperature orthorhombic Pca2₁ phase, while the resistivity measurements clearly show the first order semiconductor-semiconductor transition at the same temperature. This transition is accompanied by charge-ordering as it is confirmed by splitting of charge-sensitive vibrational modes observed in the Raman and infrared spectra. The horizontal stripe charge-order pattern is suggested based on the crystal structure, band structure calculations, and optical spectra.