Unusual 2k_F CDW state with enhanced charge ordering in β-(BEDT-TTF)₂AsF₆ and PF₆

X-ray superstructure analyses of the 2k_f CDW states for two-dimensional organic compounds β-(BEDT-FTF)₂PF₆ and AsF₆ were presented Molecular ionicity of BEDT-TTF was evaluated with both intramolecular bond lengths and molecular shape. Strong bending of quasi-neutral BEDT-TTF molecules coupled with the enhanced amplitude of charge modulation was observed Conformational ordering of the terminal ethylenes of BEDT-TTF molecules and dimerization between quasi-ionic BEDT-TTF molecules were also observed These characteristic indicates this CDW is not Peierls-Frohlich type but coupled with the internal degree of the molecular shape and possibly the intersite Coulomb repulsion.