Resonance Raman investigations of the symmetric stretching mode ofI3−anions inαandβphases of di-bis(ethylenedithio)tetrathiafulvalene tri-iodide

The resonance Raman spectra of ${\mathrm{I}}_3$ ${\mathrm{}}^{\mathrm{-}}$ anions in the organic superconductor β-(BEDT-TTF${)}_2$ ${\mathrm{I}}_3$ [BEDT-TTF represents bis(ethylenedithio)tetrathiafulvalene] were studied and discussed in comparison with the nonsuperconducting phase of α-(BEDT-TTF${)}_2$ ${\mathrm{I}}_3$. In both modifications at low temperature a splitting of the Raman mode assigned to the symmetric stretching mode of the ${\mathrm{I}}_3$ ${\mathrm{}}^{\mathrm{-}}$ anions was discovered. The splitting of this mode in the β-(BEDT-TTF${)}_2$ ${\mathrm{I}}_3$ 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 $T_c$=1.3 K into a more ordered and symmetric structure which becomes superconducting at $T_c$=8.1 K.