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 o f the bis-(ethylenedithio)tetrathiafulvalene (ET) complexed with perrhenate class [(ET)(2)(ReO4), alpha-(ET)(3)(ReO4)(2), beta-(ET)(3)(ReO4)(2)] of or ganic molecular solids have been measured as a function of temperature. For the compounds studied, the spectra are highly anisotropic, with data in th e good conductivity polarization dominated by totally symmetric electron-ph onon activated A(g) modes of ET. The low-temperature phase of each material is characterized by the opening of a semiconducting gap and vibronic symme try breaking, as evidenced by vibrational fine structure in varying degrees . The nature and splitting of the 900-cm(-1) perrhenate mode through the ph ase-transition temperatures implies that anion ordering drives the phase tr ansition in the two 3:2 phases whereas the 82-K metal --> insulator transit ion in (ET)(2)(ReO4) is a weak structural modification in which anion reord ering plays a minimal role. Correlation effects id this set of compounds ar e important, suggesting that a Hubbard gap picture may be appropriate for i nterpreting the low-lying electronic structure in these organic molecular s olids. [S0163-1829(99)01326-0].