Temperature dependence of structure and phonons of α- and β-TTF crystals

Raman phonon spectra of tetrathiafulvalene (TTF) crystals in their monoclinic (α-TTF) and triclinic (β-TTF) polymorphic modifications have been recorded as a function of temperature in the 30 to 130 cm−1 range. Along with other model potentials, the set of parameters already adopted for the study of ionic and neutral forms of bis-ethylen-dithio-tetrathiafulvalene (BEDT-TTF) has been used to compute, within a quasi-harmonic lattice dynamics framework, the crystallographic structures and the phonon frequencies of the two polymorphs as a function of temperature with good results. Introduction of the internal vibrational degrees of freedom of the TTF molecules in the crystals improves the computational approach and proves to be a valuable tool to judge the importance of the coupling between lattice phonons and intramolecular vibrations.