A novel metal–insulator transition in (EDO-TTF)2X (X=PF6, AsF6)

Single crystals of cation radical salts of ethylenedioxy-tetrathiafulvalene (EDO), (EDO)2X (X=PF6, AsF6, ClO4) were prepared by electrocrystallization. Metal–insulator (MI) transitions with big molecular deformation were observed in (EDO)2PF6 at 280 K and in (EDO)2AsF6 at 268 K. Their crystal structures are almost the same at room temperature (RT), and in both cases, the calculated band structures show quasi-one-dimensional electronic nature. The magnetic susceptibility of (EDO)2PF6 and (EDO)2AsF6 showed the thermal hysteresis around 280 and 268 K, respectively, indicating the MI transition being the first-order. Their crystal structures in the insulating phase are isostructural to each other and to that of (EDO)2ClO4 at RT. The structural analysis of these salts revealed that the mechanisms of the MI transitions observed in PF6 and AsF6 salts are the same, which show the cooperative nature of Peierls, charge-ordering (CO), and order–disorder (OD) one.