We present the site-charge distribution in the charge-ordered and metallic states of $\alpha \text{−}{\left(\text{BEDT-TTF}\right)}_2{\text{I}}_3$, based on the assignment of the $\text{C}\text{C}$ stretching modes ${\nu }_2$, ${\nu }_3$, and ${\nu }_{27}$ of $\alpha \text{−}{\left(\text{BEDT-TTF}\right)}_2{\text{I}}_3$ with the aid of ${}^{13}\text{C}$- 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 $\alpha \text{−}{\left(\text{BEDT-TTF}\right)}_2{\text{I}}_3$ with those of isostructural $\alpha \text{−}{\left(\text{BEDT-TTF}\right)}_2{\text{NH}}_4\text{Hg}{\left(\text{SCN}\right)}_4$, we propose a thermally activated fluctuation of charge order in the metallic phase of $\alpha \text{−}{\left(\text{BEDT-TTF}\right)}_2{\text{I}}_3$. This fluctuation was considerably suppressed above 0.65 GPa. The optical conductivity in the metallic phase shows no Drude response.

• Received 2 March 2010

DOI:https://doi.org/10.1103/PhysRevB.82.075134