Infrared studies of low-temperature symmetry breaking in the perrhenate family of ET-based organic molecular conductors

The polarized infrared and optical reflectance spectra of several members of the bis-(ethylenedithio)tetrathiafulvalene (ET) complexed with perrhenate class [(ET)(2)(ReO4), alpha-(ET)(3)(ReO4)(2), beta-(ET)(3)(ReO4)(2)] of organic molecular solids have been measured as a function of temperature. For the compounds studied, the spectra are highly anisotropic, with data in the good conductivity polarization dominated by totally symmetric electron-phonon activated A(g) modes of ET. The low-temperature phase of each material is characterized by the opening of a semiconducting gap and vibronic symmetry breaking, as evidenced by vibrational fine structure in varying degrees. The nature and splitting of the 900-cm(-1) perrhenate mode through the phase-transition temperatures implies that anion ordering drives the phase transition in the two 3:2 phases whereas the 82-K metal --> insulator transition in (ET)(2)(ReO4) is a weak structural modification in which anion reordering plays a minimal role. Correlation effects id this set of compounds are important, suggesting that a Hubbard gap picture may be appropriate for interpreting the low-lying electronic structure in these organic molecular solids. [S0163-1829(99)01326-0].