The very first low-frequency infrared absorption and Raman scattering spectra of TTF-CA are presented. Several studies have been performed as a function of temperature at atmospheric pressure. In the neutral phase by infrared absorption, we have emphasized the critical behaviour of an antisymmetric lattice mode (located around at 150 K) which shows the displacive nature of the neutral-to-ionic phase transition of TTF-CA at atmospheric pressure. In the ionic phase, the Raman scattering study has permitted confirmation of this result, the soft mode being located around at 16 K. Besides, the Raman study of several totally symmetric internal modes has confirmed the clearly first-order character of this transition. Finally, this study permits an estimation of the e - mv (electron-molecular vibrations) coupling constants in the neutral and ionic phases.