The correlation of lattice phonon frequencies with the superconducting transition temperatures of some (BEDT-TTF) salts

The far-infrared powder absorption spectra, between 10 cm-1 and 350 cm-1, of seven BEDT-TTF salts that exhibit superconductivity at or near ambient pressure, have been measured as a function of temperature, between 5 K and 300 K. The temperature dependences of the peak frequencies, along with the measured deuterium isotopic frequency shifts, separated the lattice from the internal modes. The frequencies of the observed lattice modes were found to increase with Tc in the three κ-phase salts in agreement with the reduced volume of the unit cell, and to have a mixed response in the three β-phase compounds. Furthermore, the general range and magnitude of the frequencies in the higher-Tc κ-phase compounds were found to be larger than in the lower Tc β-phase compounds. Consequently, no evidence was found for a correlation between higher values of Tc and a softer lattice in these organic superconductors