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, α′′-(BEDT-TTF)2Rb2xCo(SCN)4 (α′′-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 π-electron band filling between those of α′′-(BEDT-TTF)2CsHg(SCN)4 and α′′-(BEDT-TTF)2K1.4Co(SCN)4. α′′-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 two-dimensional 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 spin-charge coupling.