Infrared study of electron-molecular vibration interactions and phase transitions in the organic conductors (TMTTF)2X (X = BF−4, ClO−4, and PF−6) and TMTTF-bromanil

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Abstract
The infrared (4000–200 cm−1) absorption spectra of powder samples of (TMTTF)2X (X = BF−4, ClO−4, and PF−6) and of TMTTF-bromanil at temperatures 300 ≳T≳15 K are investigated. Polarized absorption spectra of oriented polycrystalline films of the bromanil complex in the same temperature range are also reported. Spectral features originated by the electron molecular vibration (EMV) interactions involving some totally symmetric modes of TMTTF are identified for all the compounds. For the case of the (TMTTF)2X salts, the intensity of the broad vibronic absorptions has been related to the degree of dimerization of the donor stacks. Analysis of their temperature evolution and comparison with the known behavior of the magnetic and electrical properties suggest that these systems may be viewed as strongly correlated magnetic semiconductors. The role of the commensurate anion sublattice in opening the semiconducting gap and suppressing the (4kF) Peierls instability is stressed. Possible similarities between the electronic properties of the TMTTF salts and of the bromanil complex are discussed. Analysis of vibronic absorptions of (TMTTF)2BF4 according to a ’’dimer charge oscillation model’’ yields semiquantitive estimates of the linear EMV coupling constants of TMTTF which result to be substantially similar to those previously reported for unsubstituted TTF.