Deviations from half-filling and charge ordering tendencies in β''-type purely organic charge transfer salts

The β''-(BEDT-TTF)₂X class of charge-transfer salts with sulfonate anions X^- forms a purely organic family with metallic and superconducting members. The X^-=SF₅CH₂CF₂SO₃ anion is an especially interesting compound which superconducts and for which the spatially modulated Fulde Ferrell Ovchinnikov Larkin (FFLO) superconducting order parameter has been reported. The X^-=SF₅CHFCF₂SO₃ compound is known for a metal insulator transition which occurs as function of temperature. Other members of this family of materials have X^-=SF₅CHFSO₃, X^-=SF₅CH₂SO₃ and X^-=CF₃CH₂SO₃ anions.
We investigate this class of compounds with all electron density functional theory calculations. We carefully compare electronic structures and Fermi surfaces. We find significant deviations from half-filling which help to explain the results of quantum oscillation experiments. We compare to the electronic structure of β''-(BEDT-TTF)₂X materials with inorganic anions X^-. Analysis of the tendencies to charge charge order at the DFT level yield interesting differences between the materials.