Ferroelectric Polarization in α-(ET)2I2Br Studied by Second-Harmonic Generation Microscopy

Raman spectrum and second-harmonic generation measurements have been performed for an organic conductor ?-(ET)2I2Br [ET: bis(ethylenedithio)tetrathiafulvalene]. The Raman spectrum showed splitting of a charge sensitive mode at low temperatures, illustrating that the metal-to-insulator transition of the compound is ascribed to charge ordering. Critical appearance of the second-order nonlinear optical signal below the transition temperature manifests that the inversion symmetry of the crystal is annihilated by the charge ordering to generate permanent electric polarization. This is the second example of the verification of ferroelectric transition in an organic conductor.