Optical Studies of Quasi-Two-Dimensional Organic Metals (EDT–TTF)4[Hg3I8]1–x (x = 0 and 0.027)—Comparative Analysis within the Phase Phononsmodel

Reflection spectra of quasi-two-dimensional organic conductors (EDT–TTF)₄[Hg₃I₈]_1–x, are considered, which, depending on the composition, demonstrate different phase transitions: x = 0.027—metal–superconductor with T_c = 8.1 K, x = 0—metal–insulator at a temperature of T < 35 K. The polarized reflection spectra were analyzed using the “phase phonons” model, which takes into account the electronic-vibrational interaction. Using this model, the optical functions of both compounds were adequately described and the energy parameters of the π-electron system were obtained. It is shown that for the compound with x = 0.027 these parameters change monotonically with temperature, while for x = 0 they show a jump at the phase transition temperature. A comparison of the model parameter Δ (the value of the periodic potential) for crystals with different stoichiometric coefficients x = 0 and 0.027 was carried out and an assumption was made about the dependence of the periodic potential on the average charge of one EDT molecule.