Infrared and Raman evidence for the charge-ordering inβ″−(BEDT−TTF)3(ReO4)2

We present the infrared and Raman spectra of ${\ensuremath{\beta}\phantom{\rule{0.1em}{0ex}}}^{\ensuremath{''}}{\text{\ensuremath{-}}(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{3}{{(\mathrm{ReO}}_{4})}_{2}$ $(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF}=\mathrm{bis}\text{\ensuremath{-}}\text{ethylenedithio}\text{\ensuremath{-}}\text{tetrathiafulvalene})$ measured at various temperatures. We investigated the three $\mathrm{C}\mathrm{C}$ stretching modes, ${\ensuremath{\nu}}_{2},{\ensuremath{\nu}}_{27}$, and ${\ensuremath{\nu}}_{3}$. The infrared and Raman spectra discontinuously changed at $80\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Below $80\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, for example, the doublet ${\ensuremath{\nu}}_{2}$ bands split into three peaks, the doublet ${\ensuremath{\nu}}_{27}$ also split into three, and a mutual exclusion rule for the vibronic ${\ensuremath{\nu}}_{3}$ modes in the infrared and Raman spectra was broken. This symmetry lowering was consistent with the result of x-ray crystal structure analyses conducted at $100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and $22\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The site charges in the unit cell estimated from the splitting of ${\ensuremath{\nu}}_{2}$ were $+{0.7}_{3},+{0.7}_{3}$, and $+{0.5}_{3}$ in the metallic phase, and they were $+0.2,+{0.8}_{5}$, and $+{0.9}_{5}$ in the insulating phase. The metal-insulator transition of this compound was characterized as the charge-ordering transition originated from the localization of charge due to Coulomb interactions.