Nonuniform site-charge distribution and fluctuations of charge order in the metallic state of α-(BEDT-TTF)2I3

We present the site-charge distribution in the charge-ordered and metallic states of alpha-(BEDT-TTF)(2)I-3, based on the assignment of the C=C stretching modes nu(2), nu(3), and nu(27) of alpha-(BEDT-TTF)(2)I-3 with the aid of C-13- and deuterium- substituted compounds. The nonuniform site charges in both the metallic and insulating phases were consistent with those determined by a recent x-ray diffraction study. Comparing the line shapes of the charge-sensitive vibrational modes of alpha-(BEDT-TTF)(2)I-3 with those of isostructural alpha-(BEDT-TTF)(2)NH4Hg (SCN)(4), we propose a thermally activated fluctuation of charge order in the metallic phase of alpha-(BEDT-TTF)(2)I-3. This fluctuation was considerably suppressed above 0.65 GPa. The optical conductivity in the metallic phase shows no Drude response.