The crystal structure and lattice phonons of the $(\mathrm{BEDT}-\mathrm{TTF}{)}_2{\mathrm{I}}_3$ superconducting $\beta$ phase (where BEDT-TTF is bis-ethylen-dithio-tetrathiafulvalene) are computed and analyzed by the quasiharmonic lattice dynamics (QHLD) method. The empirical atom-atom potential is that successfully employed for neutral BEDT-TTF and for nonsuperconducting $\alpha -(\mathrm{BEDT}-\mathrm{TTF}{)}_2{\mathrm{I}}_3.$ The rigid molecule approximation has to be relaxed by allowing mixing between lattice and low-frequency intramolecular vibrations. Such a mixing is found to be essential to account for the specific heat measurements, and also yields good agreement with the observed Raman and infrared frequencies. The crystal structure and its temperature and pressure dependence are also properly reproduced, though the effect of the mixing is less important in this case. From the eigenvectors of the low-frequency phonons we calculate the electron-phonon coupling constants due to the modulation of charge transfer (hopping) integrals. The charge transfer integrals are evaluated by the extended Hückel method applied to all nearest-neighbor BEDT-TTF pairs in the ab crystal plane. From the averaged electron-phonon coupling constants and the QHLD phonon density of states we derive the Eliashberg coupling function ${\alpha }^2\left(\omega \right)F\left(\omega \right),$ which compares well with that experimentally obtained from point contact spectroscopy. The corresponding dimensionless coupling constant $\lambda$ is found to be $\sim \mathrm{0.4.}$

• Received 17 March 2000

DOI:https://doi.org/10.1103/PhysRevB.62.14476