JOURNAL DE PHYSIQUE Colloque C3, supplément au n°6, Tome 44, juin 1983 page C3-1453 INFRARED STUDIES OF PHASE TRANSITIONS IN SINGLE- AND TWO-CHAIN ORGANIC CHARGE TRANSFER CONDUCTORS R. Bozio, C. P e c i l e and P. Tosi Institute of Physiaal Chemistry, The University, 2 Via Loredan, Padova, Italy Résumé - Les absorptions vibroniques infrarouge de MEM(TCNQ) 2 sont discutées en relation avec les transitions de Peierls électroniques et de spin, et avec le désordre structural. Le rôle des chaînes donneur et accepteur dans la t r a n s i t i o n de P e i e r l s de TMTTF-TCNQ et HMTTF-TCNQ e s t démontré. A b s t r a c t - The v i b r o n i c i n f r a r e d a b s o r p t i o n s o f MEM(TCNQ)2 a r e d i s c u s s e d in relation to the electronic- and spin-Peierls transitions and to the structural disorder. The rôles of the donor and acceptor chains in the Peierls transi- t i o n s o f TMTTF-TCNQ and HMTTF-TCNQ a r e d e m o n s t r a t e d . I - ONE-CHAIN CONDUCTORS: MEM(TCNQ)2 VS. (TMTTF) 2 X. The température dependence of the infrared spectra of (TMTTF)2X c o n d u c t o r s h a s been interpreted 1 as évidence that, due to strong electron-electron interactions and to the effect of the counter-ions, thèse Systems are magnetic semiconductors at room température. Correspondingly, their low température phase transitions have been thought to be of the spin-Peierls type. Owing t o an e f f e c t i v e decoupling of the elec_ tronic spin and translational degrees of freedom, the effect of thèse transitions on the infrared spectra, as well as on the electrical conductivity, is very small. MEM(TCNQ) 2 has been considered as a model System to verify thèse ideas. In fact it is a 1/4-filled System belonging to the large U/t limit. It undergoes a 2kp (di- merization) electronic-Peierls transition and a 4k F (tetramerization) spin- Peierls 3 transition at widely separated températures (335 K and 19 K respectively). The infrared reflection spectra of MEM(TCNQ)2 at room température and 350 K have been already reported and discussed with respect to the strength of the electron- molecular vibration (e-mv) interactions.^ Here, we présent a température dépendent study of the intensity and bandwidth of the vibronic (e-mv) absorptions in the tem- pérature range 380-6 K. The results will be discussed in relation to the occurrence of both the électron- and spin-Peierls transitions and to the température dependence of the structural disorder in the MEM + sublattice. 5 MEM(TCNQ)2 was p r e p a r e d by m i x i n g h o t s o l u t i o n s o f TCNQ and MEM i o d i d e i n a c e t o - nitrile. Spectra of powders suspended in minerai oil on Csl Windows were recorded using the instrumentation and methods previously described. Our room température powder spectrum agrées with that reported by Rice et al. except for the fact that, due to a lower spectral resolution, same détails of the band structures and weak features are lost in the latter spectrum. Table I collects the infrared frequencies observed in the MEM(TCNQ)2 spectrum at 15 K. The counter-ion absorptions are sorted out by comparison with the spectrum of MEM iodide (also reported in Table I) and the remaining frequencies are assigned to spécifie TCNQ modes. The most prominent features of the spectrum (whose frequency is underlined in Table I) are attributed to a vibronic effect (dimer charge oscillations ) originated by the coupling of the unpaired électrons with the totally symmetric (a g ) intramolecular vibrations o f TCNQ. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol/1983041