Electronic properties of new quasi-two-dimensional organic conductors (BEDO-TTF)5[MHg(SCN)4]2 (M=Rb, Cs) studied by IR reflectance spectroscopy at temperatures down to 10 K

Polarized reflectivity and optical conductivity spectra of new quasi-two-dimensional organic conductors (BEDO-TTF)(5)[MHg(SCN)(4)](2), where M = Rb, Cs, are studied over the spectral range 700-6500 cm(-1) at temperatures between 10 and 300 K. The Rb salt shows metallic behavior over the whole measured spectral and temperature range. For the Cs salt an unusual anisotropy of changes in the spectra of the conducting plane is observed with decreasing temperature. At low temperature this salt exhibits metallic behavior in the direction perpendicular to the BEDO-TTF stacks, but non-metallic behavior in the direction parallel to the stacks. This anisotropy is confirmed by dc conductivity measurements. It is supposed that the nonmetallic behavior is due to a dimerization of the BEDO-TTF molecules in the stacks at low temperature. The appearance of new bands in the vibrational spectra gives evidence for this interpretation. The ordered structure of the anion layers of the Cs salt is a reasonable cause of the dimerization.