Charge Order-Disorder Phase Transition in alpha '-[Bis(ethylenedithio)tetrathiafulvalene](2)IBr2 [alpha '-(BEDT-TTF)(2)IBr2]

We present the anomalies at similar to 200 K in electrical resistivity, transmittance and reflectance in the far-infrared region, and the magnetic susceptibility of alpha'-(BEDT-TTF)(2)IBr2 [BEDT-TTF = bis(ethylene-dithio)tetrathiafulvalene]. This phase transition is investigated by infrared, Raman, and far-infrared spectroscopies. Through the spectroscopic investigation, we found that the positive charges on BEDT-TTF molecules are strongly localized owing to a strong Coulomb interaction in both low-temperature (LT) and high-temperature (HT) phases. We show the spectroscopic data that suggest the charge order in the LT phase and the charge disorder in the HT phase. The phase transition at similar to 200K in alpha'-(BEDT-TTF)(2)IBr2 is the charge order-disorder phase transition not accompanied by Structural changes. On the basis of the localized Picture, we qualitatively interpreted the experimental results above and below the phase transition. The effect of hydrostatic pressure was investigated by electrical resistivity Measurement and Raman spectroscopy. The charge-ordered State is Successively Suppressed by applying hydrostatic pressures of up to 1.2 GPa. The resistivity and Raman spectrum qualitatively change above 1.4 GPa. We speculate that the phase transition changes the character from a charge order-disorder to a metal-insulator.