Charge ordering inθ−(BEDT−TTF)2RbZn(SCN)4studied by vibrational spectroscopy

Abstract

Charge distribution in the insulating phase of $\theta -(\mathrm{BEDT}-\mathrm{TTF}{)}_2\mathrm{RbZn}(\mathrm{SCN}{)}_4$ [BEDT-TTF denotes bis(ethylenedithiolo)tetrathiafulvalene] is investigated using the polarized Raman and infrared (IR) spectroscopy. Both of the Raman and IR spectra showed a multiple-peaks pattern below metal-insulator transition temperature. We assigned the carbon-double-bond $\left(\mathrm{C}=\mathrm{C}\right)$ stretching-modes based on the isotope shift in the ${}^{13}\mathrm{C}$-substituted sample, and performed a factor-group analysis for the polarized IR and Raman spectra. These results proved that charge disproportionation (CD) takes place in the insulating phase, and that the spatial pattern of the charge ordering agrees with the symmetry determined by the x-ray diffraction. The CD ratio was estimated from the frequency of a ring $\mathrm{C}=\mathrm{C}$ stretching mode that is weakly coupled with electronic excited states. A systematic way to investigate the CD phenomenon based on vibrational spectroscopy is discussed.