Optical studies of quasi-two-dimensional organic metals (EDT–TTF)$_4$[Hg$_3$I$_8$]$_{1-x}$ ($x$ = 0 and 0.027) – comparative analysis within the phase phononsmodel

Reflection spectra of quasi-two-dimensional organic conductors (EDT–TTF)$_4$[Hg$_3$I$_8$]$_{1-x}$, 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 $\pi$-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 $\Delta$ (the value of the periodic potential) for crystals with different stoichiometric coefficients $x$ = 0 and $x$ = 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.