Unusual transport and EPR properties of the (BET-TTF)-XF6 salts (X=P, As)

Bis(ethylenethio)tetrathiafulvalene (BET-TTF) forms with the octahedral counterions PF6− and AsF6− charge transfer salts with different composition. We focus on three of these salts: (i) a new mixed-valence salt, (BET-TTF)x(AsF6)y, which has metallic behaviour down to 80 K and whose EPR signal splits into two lines on cooling; (ii) two isostructural salts, (BET-TTF)(XF6)0.7±δ (X=P, As), which reveal three possible phase transitions as temperature decreases. Optical and transport properties, EPR studies, as well as the crystal and electronic band structures are reported and discussed.