Charge-ordering phase transition in α'-(BEDT-TTF)2IBr2

The phase transition found in the electrical resistivity of α'-(BEDT-TTF)2IBr2 was investigated by observing the charge-sensitive phonon modes of BEDT-TTF. The low-temperature phase was elucidated to be a charge-ordered state with large amplitude. The pattern of charge-order was a horizontal or diagonal stripe, which requires the breaking of inversion symmetry. The breaking of symmetry was confirmed by observing the asymmetric stretching mode of IBr2-. The symmetry breaking occurs primarily at -200 K rather continuously. It was proposed in the high-temperature phase that the nearly localized charges are dynamically fluctuating individually or collectively with a fluctuation rate much slower than 55 cm-1.