Metal-insulator phase transition in the delta-(BEDT-TTF)(4)[2,6-anthracene-bis(sulfonate)]center dot 4H(2)O studied by infrared spectroscopy

Temperature dependence of polarized infrared (IR) reflectance spectra of two-dimensional weakly dimerized organic conductor delta-(BEDT-TTF)(4)[2,6-anthracene-bis(sulfonate)]center dot 4H(2)O with 1/4-filled band has been measured to investigate the metal-insulator phase transition at T-MI = 85K linked to charge ordering (CO). Vibrational bands related to C=C stretching vibrations of the BEDT-TTF donor molecule give evidence of the CO and charge fluctuations both in the metallic and insulating phases above and below 85 K, respectively. Considerable modifications of the vibrational features due to electron-molecular vibration coupling are also observed. The phase transition is associated with the reorganization of hydrogen OH center dot center dot center dot O bonds in anion layers between the sulfonate moieties of the anion and the trapped water molecules, which is transmitted to conducting BEDT-TTF layers through weaker C-H center dot center dot center dot O bonds. The analysis of electronic bands confirms the presence of strong electronic correlations in the material. Two electronic absorptions are observed in the mid-IR region, attributed to transitions between Hubbard bands and transitions inside BEDT-TTF dimers. A low-energy electronic absorption in the far-IR region is attributed to collective excitations of the short-range charge order. Below 85 K, the opening of a charge gap of similar to 24 meV is observed both parallel and perpendicular to the BEDT-TTF stacking direction. The spectroscopic data indicate that the CO phase