Optical study of the transformation of the α-phase of (BEDT-TTF)2I3 into the superconducting αt-phase (Tc = 8K)

Abstract

The tempering of α-(BEDT-TTF)2I3 crystals above 70°C for several days leads to the transformation into the superconducting αt-phase (Tc = 8 K at ambient pressure). This process was observed in very small crystals by polarizing microscope techniques in the visible and near infrared spectral range. Premature termination of the tempering yields to crystals containing both phases in the same crystal. This allows direct comparison of the optical properties of the both phases. The orientation of the polarizability tensor and the transmission spectra parallel to the principal axes were measured. The spectra are similar to the spectra of the β-phase. But the indicatrix orientation rather indicates a new phase than the β-phase.