Single-crystal infrared spectra of the synthetic metal—solid electrolyte hybrid salts (ET)2Cu5I6 and (ET)3AgxI8, 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)tetrathiafulvalene, (ET)2Cu5I6 and (ET)3AgxI8, showing the properties of both electron conductors and solid electrolytes, were studied. Main spectral features of the room-temperature spectra of the (ET)2Cu5I6 crystals strongly resemble those of the β-(ET)2X crystals (X = I3,AuI2,I2Br,PF6), having similar arrangement of ET molecules in the conducting layer. However, temperature dependence of the (ET)2Cu5I6 reflectance is found to be in strong contrast with those of metallic β-(ET)2X. Upon cooling, the interband transition exhibits a strong spectral weight loss and a shift towards lower energies, probably due to the formation of a gap on some part of the Fermi surface. The infrared spectra of (ET)3AgxI8 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 positions of interference reflectance maxima show anomalies at temperatures 180 and 120 K confirming thus the existence of phase transitions in the (ET)3AgxI8 salt.