Optical properties of α′-(BEDT-TTF)2IBr2

The molecular semiconductor α′-(BEDT-TTF)2IBr2 is known to undergo structural and magnetic phase transitions at around 200 K and 60 K, respectively. In order to elucidate the mechanism of these phase transitions, measurements of the polarised reflectance spectra and Raman spectra were carried out at low temperatures. In the reflectance spectra, the spectral change in the infrared region was observed at around 200 K. In the Raman spectra, the band at 1480 cm−1 splits with decreasing temperature. No change was not observed for both measurements at around 60 K. We point out the possibility that the BEDT-TTFs may posses different charges in this salt.