Crystal structure of charge ordered compound θ-(BEDT-TTF)2RbCo(SCN)4 at low temperatures

The crystal structure of theta-(BEDT-TTF)(2)RbCo(SCN)(4) in its insulating state below T-MI (approximately 200 K) was investigated by X-ray diffraction experiments. The transfer integrals between the BEDT-TTF molecules were also calculated from the structural data, and the ionicities of BEDT-TTF molecules were estimated from the intramolecular bond length distribution. A large modulation of transfer integrals and charge disproportionation between BEDT-TTF molecules were found. The ratio of ionicities was 0 to +0.2 : +0.8 to +1.0. These results were compared with thoes of the analogous salt theta-(BEDT-TTF)(2)RbZn(SCN)(4).