THE NEUTRAL-TO-IONIC PHASE-TRANSITION OF TTF-CA - A RAMAN AND INFRARED STUDY VERSUS TEMPERATURE AT ATMOSPHERIC-PRESSURE

The very first low-frequency infrared absorption and Raman scattering spectra of TTF-CA are presented. Several studies have been performed a s a function of temperature at atmospheric pressure. In the neutral ph ase by infrared absorption, we have emphasized the critical behaviour of an antisymmetric lattice mode (located around 30 cm(-1) at 150 K) w hich shows the displacive nature of the neutral-to-ionic phase transit ion of TTF-CA at atmospheric pressure. In the ionic phase, the Raman s cattering study has permitted confirmation of this result, the soft mo de being located around 105 cm(-1) at 16 K. Besides, the Raman study o f several totally symmetric internal modes has confirmed the clearly f irst-order character of this transition Finally, this study permits an estimation of the e-mv (electron-molecular vibrations) coupling const ants in the neutral and ionic phases.