Structure and Physical Properties of a New 1:1 Cation-Radical Salt, ζ-(BEDT-TTF)PF₆

The synthesis, structural characterization, electrical transport, optical, and magnetic properties of a new cation-radical salt, bis(ethylenedithio)tetrathiafulvalene hexafluoro-phosphate, ζ-(BEDT-TTF)PF₆, are reported. The X-ray structure was determined at 298 K and at 173 K. The primary difference at the two temperatures is that the PF₆^- anions are disordered at 298 K and ordered at 173 K. At 298 K the salt is in the triclinic space group P1̄ with crystallographic parameters a = 6.2793(1) Å, b = 7.3680(1) Å, c = 9.9006(1) A, α = 92.542(1)°, β= 93.255(1)°, γ = 98.763(1)°, vol = 451.33(1) ų. At 173 K the salt retains the same space group with parameters a = 6.2293(1) Å, b = 7.2633(2) Å, c = 9.8662(2) Å, α = 92.898(1)°, β= 93.592(1)°, γ = 97.914(1)°, vol = 439.95(2) ų. Despite a crystal structure with uniform and segregated layers of donors and counterions, the room-temperature dc conductivity is rather low, ∼1.1 × 10^-4 Ω^-1 cm^-1, and decreases rapidly upon cooling. Polarized reflectance measurements along the a axis of ζ-(BEDT-TTF)PF₆ provide evidence that the material is a Mott-Hubbard insulator with an effective on-site Coulomb interaction U_eff of 0.82 eV and a transfer matrix element t of 0.21 eV. An analysis of the infrared A_g modes yields a dimensionless electron-molecular-vibration coupling constant λ ∼ 0.1. When the temperature is lowered, the activation energy for transport, the far-infrared phonons, and the magnetic susceptibility χ all exhibit an anomaly at ∼250 K which is related to the order/disorder transition of the PF₆^- counterions. In the low-temperature phase, the spectral weight of the three lowest A_g modes changes gradually, suggesting that the phase transition is close to second order. Finally, an exponential drop of χ occurs below 40 K, indicating the opening of a gap in the spin excitation spectrum. We suggest that this magnetic phase transition has structural aspects and can probably be associated with a spin-Peierls transition.