Crystal Structure of Charge Ordered Compound θ-(BEDT-TTF)₂RbCo(SCN)₄ at Low Temperatures

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The crystal structure of θ-(BEDT-TTF)₂RbCo(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 θ-(BEDT-TTF)₂RbZn(SCN)₄.