Charge Ordering and π-d Interaction in Electron-Doped 3/4-Filling Molecular System α'-(BEDT-TTF)2Rb2xCo(SCN)4 (x = 0.6).

We have investigated a newly found α'-phase bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) molecular arrangement system, namely, αAA-(BEDT-TTF)₂Rb_2xCo(SCN)4 (α'-Rb_2xCo), with localized S = 3/2 Co²⁺ spins. From X-ray structural analyses, we found that owing to the nonstoichiometric ratio of Rb ions (x = 0.6), this compound takes an intermediate value of π-electron band filling between those of αAA-(BEDT-TTF)₂CsHg(SCN)₄ and α'-(BEDTTTF)₂K_1.4Co(SCN)₄. α'-Rb_2xCo (x = 0.6) is a paramagnetic metal at room temperature and exhibits a first-order transition to an insulating state at 100 K, which is the lowest among the three compounds. The results of the twodimensional optical conductivity spectra and peak splitting of charge-sensitive ν27 molecular vibrational modes indicate the phase transition to a state with a gap of 600 cm-1, which is attributed to charge ordering consisting of at least four differently charged BEDT-TTF molecules. The π-spin susceptibility suddenly decreases at the transition temperature, and another anomaly can be seen at about 40 K. Given that π-spin disappears at 100 K (i.e., formation of a spin-singlet state), it is difficult to systematically explain other physical properties as a whole at low temperatures. However, by simply assuming the presence of π spins, we can account for all the observed results without contradiction. We also discuss anomalies in the dielectricity of π electrons under magnetic fields mediated by π-d interactions and the spincharge coupling [ABSTRACT FROM AUTHOR]