SINGLE-CRYSTAL INFRARED-SPECTRA OF THE SYNTHETIC METAL-SOLID ELECTROLYTE HYBRID SALTS (ET)(2)CU5I6 AND (ET)(3)AGXI8 WHERE ET IS BIS(ETHYLENEDITHIO)TETRATHIAFULVALENE

Infrared reflectance and transmittance spectra (700 to 6500 cm(-1)) of two charge transfer salts of the bis(ethylenedithio) tetrathiafulvale ne, (ET)(2)Cu5I6 and (ET)(3)AgxI8, showing the properties of both elec tron conductors and solid electrolytes, were studied. Main spectral fe atures of the room-temperature spectra of the (ET)(2)Cu5I6 crystals st rongly resemble those of the beta-(ET)(2)X crystals (X= I-3, AuI2, I2B r, PF6), having similar arrangement of ET molecules in the conducting layer. However, temperature dependence of the (ET)(2)Cu5I6 reflectance is found to be in strong contrast with those of metallic beta-(ET)(2) X. Upon cooling, the interband transition exhibits a strong spectral w eight loss and a shift towards lower energies, probably due to the for mation of a gap on some part of the Fermi surface. The infrared spectr a of (ET)(3)AgxI8 crystals are essentially different, despite similar molecular arrangement in the crystal structure. We suggest that it is mainly due to the essentially different band-filling factor. The posit ions of interference reflectance maxima show anomalies at temperatures 180 and 120 K confirming thus the existence of phase transitions in t he (ET)(3)AgxI8 salt. (C) 1998 Elsevier Science S.A; All rights reserv ed.