Structural Changes in Superconducting BEDT-TTF Salts

Abstract

Relations are investigated between electrical and structural properties in organic superconductors bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) salts. It is known that the superconductivity transition temperature T_c of β-(BEDT-TTF)₂I₃ salts is raised from 1.5 to 8 K when the sample experiences high pressures. Furthermore, at ambient pressure, an incommensurate superstructure is formed with the wave vector q= (0.08, 0.27, 0.205) below 200 K in this salt. With the hypothesis that the increase of T_c is brought about by a suppression of the growth of the superstructure, and that the superstructure is hardly formed in samples which experienced high pressures, the temperature dependence of satellite intensities is measured by X-ray diffraction in those samples. A negative result is obtained for this hypothesis. Namely the behavior of the superstructure is found to be insensitive to pressurization. However, this conclusion does not exclude the possible change of the superstructure in a small volume of crystals, which can hardly be detected by X-ray diffraction.
A salt of (BEDT-TTF)₂IBr₂ shows semiconducting properti es and its activation energy increases with decreasing temperature. It is found that a molecular dimerization occurs with temperature decreasing from room temperature. This dimerization is considered to cause the increase of the activation energy. In this salt, another structural change seems to take place near 130 K.