Charge disproportionate state of BEDT-TTF$\beta ''$ – salts

We studied C=C stretching bands of $\beta'' $-(ET)$_{3}$(ReO$_{4})_{2}$ and $\beta $-(ET)$_{4}$M(CN)$_{4}$H$_{2}$O (M=Pt, Pd and Ni) at the low temperatures below 300 K in order to verify the inhomogeneous charge distributions. A drastic spectral change was observed for $\beta ''$-(ET)$_{3}$(ReO$_{4})_{2}$ at 80 K, below which the system falls into the charge ordered state. On the other hand, a continuous spectral change was observed for $\beta ''$-(ET)$_{4}$Ni(CN)$_{4}$H$_{2}$O and $\beta ''$-(ET)$_{4}$Pd(CN)$_{4}$H$_{2}$O. $\nu $$_{2}$ for M=Ni and Pd exhibited a doublet, suggesting an inhomogeneous charge distributions. $\nu $$_{2}$ for M=Pt did not show a peak splitting, while C=C bands for the deuterium analogue exhibited a behaviour similar to those for M=Ni. Key words. Molecular conductors – organic conductors – ET salts – vibrational spectra – structural analyses – charge ordered state – mean-field calculations–isotope effect