THE INFRARED OPTICAL-PROPERTIES OF THE ALPHA-PHASE ORGANIC SUPERCONDUCTOR, ALPHA-(BEDT-TTF)2(NH4HG(NCS)4)

The title compound is the only known organic superconductor (T(c) = 0. 8 K) with an alpha-phase crystal structure, in which the conducting pl ane contains single BEDT-TTF molecules with alternating orientation, r ather than the pairs of molecules found in the higher T(c) kappa-phase compounds. We have measured the polarized reflectivity of protonated and deuterated (BEDT-TTF)2(NH4)Hg-(NCS)4, from 80 cm-1 to 8000 cm-1, a t temperatures between 6.5 K and 300 K. The overall shape of these spe ctra are discussed. The optical conductivity obtained after a Kramers- Kronig analysis shows no maximum in the mid-infrared as known from the compounds of the kappa-phase due to the lack of dimerization of the B EDT-TTF molecules. This causes an accentuation of the far-infrared int raband conductivity. The vibrational features are assigned and discuss ed.