Evidences for 1-D CDW Fluctuations and 3-D Ordering in the IR Spectra of the Organic Conductor (FA)2PF6

The broad band spectrum of optical conductivity of the quasi one-dimensional organic conductor (FA)2PF6 has been investigated in a wide temperature range including the Peierls transition temperature Tp = 180 K. The semiconducting gap value has been observed both at 10 K (∼143 meV) and at 300 K (∼80 meV). We show in the present study that in contrast to the dc conductivity the electronic part of dynamical conductivity at energies 0.02–0.06 eV is mainly driven by 1-D fluctuations in a wide temperature range and does not show any distinct crossover to 3-D behavior below Tp , while this crossover is certainly observed in the temperature dependence of the intensity of intramolecular vibrations. From the latter the temperature dependence of the order parameter for the Peierls transition has been determined to be Δ(T) ∼ (Tp — T)1/4 what is characteristic for a second order phase transition near tricritical point.