Superconductivity in polycrystalline pressed pellets of k-(BEDT-TTF)2I3

Abstract

Investigations of the transport properties of single crystals and polycrystalline pressed samples of K-( BEDT-TTF)2I3 are presented. In contrast to polycrystalline pressed samples of K-(BEDT-TTF)2Cu(NCS)2 and K-(BEDT-TTF)2Cu [N(CN)2]Br those of K-(BEDT-TTF)2I3 show bulk superconductivity indicating that the loss of superconductivity in the first two types of samples is not due to the special structure of the K-phase, but probably due to a disproportionation of Cu1+ under the anisotropic pressure during the preparation of the samples. Annealing the single crystals as well as the polycrystalline pressed samples of K-(BEDT-TTF)2I3 at 75°C for a few days results in a structural transfomation with an increase in the transition temperature for superconductivity (about 8 K for the crystals). IR-spectra indicate a transfomatlon into a structure which is identical with αt-(BED-TTF)2I3 which seems to be the thermodynamically most stable phase of the several (BEDT-TTF)2I3 phases.