Resonance Raman investigations of the symmetric stretching mode of I3- anions in α and β phases of di-bis(ethylenedithio)tetrathiafulvalene tri-iodide

Abstract

The resonance Raman spectra of I3- anions in the organic superconductor β-(BEDT-TTF)2I3 [BEDT-TTF represents bis(ethylenedithio)tetrathiafulvalene] were studied and discussed in comparison with the nonsuperconducting phase of α-(BEDT-TTF)2I3. In both modifications at low temperature a splitting of the Raman mode assigned to the symmetric stretching mode of the I3- anions was discovered. The splitting of this mode in the β-(BEDT-TTF)2I3 crystals is related to the commensurate superstructure developed below 125 K in contradiction to the α phase where the splitting is related to a crystal-field effect. It is shown that an electronic excitation of the β-phase crystals by laser light can induce a structural transformation at least in a thin layer of the surface. This transformation is observed by a disappearance of the splitting of the symmetric stretching mode. The structural change can be related to a transformation from the commensurate superstructure with a superconducting transition temperature Tc=1.3 K into a more ordered and symmetric structure which becomes superconducting at Tc=8.1 K.