Temperature dependence of structure and phonons of alpha- and beta-TTF crystals

Raman phonon spectra of tetrathiafulvalene (TTF) crystals in their monoclin ic (alpha -TTF) and triclinic (beta -TTF) polymorphic modifications have be en recorded as a function of temperature in the 30 to 130 cm(-1) range. Alo ng with other model potentials, the set of parameters already adopted for t he study of ionic and neutral forms of bis-ethylen-dithio-tetrathiafulvalen e (BEDT-TTF) has been used to compute, within a quasi-harmonic lattice dyna mics framework, the crystallographic structures and the phonon frequencies of the two polymorphs as a function of temperature with good results. Intro duction of the internal vibrational degrees of freedom of the TTF molecules in the crystals improves the computational approach and proves to be a val uable tool to judge the importance of the coupling between lattice phonons and intramolecular vibrations.