Synthesis and physical characterization of new d-transition metal fluoride GICSs

Abstract

Several routes have been carried out to prepare new GICs containing transition metal fluorides : the gas-solid reaction is employed with gaseous RhF 5 for instance, whereas the liquid-solid reaction in anhydrous hydrogen fluoride saturated with fluoride ions is used with Cr, Rh and Au fluorides. The obtained compounds crystallize in stage-1 to stage-3 depending on the nature of host graphite. The thickness of the intercalate, which can be deducted from the repeat distance (I c) value, generally ranges from 4.50 to 4.85 Å. On the basis of optical reflectivity experiments carried out at Kyoto University, a quantitative interpretation has been proposed using the 2-D band structure model of graphite acceptor compounds (Blinowsky-Rigaux model) [1]. For Cr- and Rh-based materials, the best fitting is obtained for the following Fermi level (E F) and relaxation time (τ): C 21CrF 4.5 E F = 0.94 ± 0.03 eV; τ = 2.8·10 −14 S C 28RhF 3.3(HF 2) 1.3 EF = 0.89 ± 0.03 eV; τ = 1.2·10 −14 S The corresponding charge transfer coefficients can be deduced from the E F values: f c = 0.024 and f c = 0.021), respectively. These values can be correlated to a C-C bond length contraction: • Cr-based GIC: d c-c = 1.4165 Å → 4 x 10 −3 Å contraction • Rh-based GIC: d c-c = 1.4176 Å → 3.3 x 10 −3 Å contraction. This trend is confirmed by the shift towards higher energies observed in Raman E 2 2g peak. The oxidation state of the transition metals has been deduced from ESR, XANES, magnetic and XPS studies, whereas the coordination of Rh and Au has been shown to be octahedral and square planar respectively, using EXAFS spectroscopy.