SUPERCONDUCTIVITY IN POLYCRYSTALLINE PRESSED PELLETS OF KAPPA-(BEDT-TTF)2I3

Investigations of the transport properties of single crystals and poly crystalline pressed samples of K-(BEDT-TTF)213 are presented. In contr ast 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 superconduc tivity 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-degrees-C for a few days results in a structural transfomat ion with an increase in the transition temperature for superconductivi ty (about 8 K for the crystals). IR-spectra indicate a transfomation i nto a structure which is identical with alpha(t) - (BEDT - TTF)2I3, wh ich seems to be the thermodynamically most stable phase of the several (BEDT-TTF)2I3 phases.