Lattice distortion stabilizes the photoinduced metallic phase in the charge-ordered organic salts (BEDT-TTF)3X2 (X=ReO4, ClO4).

Photoinduced effects caused by intramolecular excitation were investigated by simultaneous optical and transport measurement in two charge-ordered organic salts, (BEDT-TTF)3X2 (X=ReO4, ClO4) [BEDT-TTF=bis(ethylenedithio)tetrathiafulvalene]. Although the two salts have the same molecular (average) charge and arrangement, they showed different photoinduced effects. A photoinduced insulator-to-metal phase transition with a metastable charge order-melting state was observed in the ReO4 salt where the charge ordered state is associated with the lattice distortion. On the other hand, no photoinduced insulator-to-metal phase transition was noted in the ClO4 salt where the charge ordered state is not accompanied by the lattice distortion. This comparative study suggested that the lattice distortion plays a key role in the stabilization of the photoinduced phase.