Charge ordering and π-d interaction in electron-doped 3/4-filling molecular system a''-(BEDT-TTF)₂Rb_2xCo(SCN)₄(x = 0.6)

We have investigated a newly found a''-phase bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) molecular arrangement system, namely, a''-(BEDT-TTF)2Rb2xCo(SCN)4 (a''-Rb2xCo), 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 p-electron band filling between those of a''-(BEDT-TTF)2CsHg(SCN)4 and a''-(BEDTTTF) 2K1.4Co(SCN)4. a''-Rb2xCo (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 100K (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 p spins, we can account for all the observed results without contradiction. We also discuss anomalies in the dielectricity of p electrons under magnetic fields mediated by p-d interactions and the spincharge coupling.