FT-IR ABSORPTION-SPECTROSCOPY OF BEDT-TTF RADICAL SALTS - CHARGE-TRANSFER AND DONOR ANION INTERACTION

IR absorption spectra from various kappa-phases of BEDT-TTF radical cation salts as well as from alpha-(BEDT-TTF)2I3 are presented. From these spectra one special mode nu27 representing the asymmetric ring C=C stretching vibration is assigned and the dependence of its frequency from the average charge on the donor molecule is given. The investigation of the metal-insulator phase transition of alpha-(BEDT-TTF)2I3 at 135 K with respect to this vibrational model nu27 gives strong indication of a charge localization in at least one of the two crystallographic different stacks. Furthermore, we correlate the frequencies of the CH2 stretching modes of a variety of BEDT-TTF radical salts with the superconducting phase transition temperatures. These frequencies are evidently not charge dependent, but significantly reflect the strength of the interaction of the donor molecule with the respective anion. It could be shown that phases with a higher T(c) of their superconducting transition exhibit a smaller red shift of these frequencies, i.e., a less attractive donor-anion interaction.