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

The synthesis, structural characterization, electrical transport, optical, and magnetic properties of a new cation-radical salt, bis(ethylenedithio)tetrathiafulvalene hexafluorophosphate, ú-(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 P1h with crystallographic parameters a ) 6.2793(1) Å, b ) 7.3680(1) Å, c ) 9.9006(1) Å, R ) 92.542(1)°, â ) 93.255(1)°, γ ) 98.763(1)°, vol ) 451.33(1) Å3. 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, R ) 92.898(1)°, â ) 93.592(1)°, γ ) 97.914(1)°, vol ) 439.95(2) Å3. Despite a crystal structure with uniform and segregated layers of donors and counterions, the roomtemperature 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 ú-(BEDT-TTF)PF6 provide evidence that the material is a Mott-Hubbard insulator with an effective on-site Coulomb interaction Ueff of 0.82 eV and a transfer matrix element t of 0.21 eV. An analysis of the infrared Ag 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 ø all exhibit an anomaly at ∼250 K which is related to the order/disorder transition of the PF6 counterions. In the lowtemperature phase, the spectral weight of the three lowest Ag modes changes gradually, suggesting that the phase transition is close to second order. Finally, an exponential drop of ø 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.