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 ca tion salts as well as from alpha-(BEDT-TTF)2I3 are presented. From the se spectra one special mode nu27 representing the asymmetric ring C=C stretching vibration is assigned and the dependence of its frequency f rom the average charge on the donor molecule is given. The investigati on of the metal-insulator phase transition of alpha-(BEDT-TTF)2I3 at 1 35 K with respect to this vibrational model nu27 gives strong indicati on of a charge localization in at least one of the two crystallographi c different stacks. Furthermore, we correlate the frequencies of the C H2 stretching modes of a variety of BEDT-TTF radical salts with the su perconducting phase transition temperatures. These frequencies are evi dently not charge dependent, but significantly reflect the strength of the interaction of the donor molecule with the respective anion. It c ould be shown that phases with a higher T(c) of their superconducting transition exhibit a smaller red shift of these frequencies, i.e., a l ess attractive donor-anion interaction.