Existence of two phases in β-(BEDT-TTF)2I3: proof by resonance Raman spectroscopy

Abstract

Among various organic metals and superconductors based on the BEDT-TTF molecule [bis(ethylenedithiololtetrathiafulvalene] and different polyhalide anions the most intensively studied is β-(BEDT-TTF)2I3. Partially it is caused by the fact that these crystals exhibit the superconductivity with the highest critical temperature reported for organic materials, but the main reason of this interest is the existence of two superconducting states in these crystals. Under ambient pressure the superconductivity in β-(BEDT-TTF)2I3 is observed below Tc≈1,3K1-4 (low-Tc phase) but after a particular pressure-temperature cycling procedure the superconductivity can be stabilized under ambient pressure at Tc≈8,1K5,6(high-Tc phase). α-(BEDT-TTF)2I3 is another modification of the salt formed hetween BEDT-TTF and I3-; at ambient pressure α-phase crystals undergo a metal-insulator phase transition at T=135K 4,7. Recently, it was discovered that by tempering α-phase crystals at about 75°C for several days crystals can be obtained with similar properties as the high-Tc state of the β-crystals 8,9 (these crystals are further denoted as α t-(BEDT-TTF)2I3).