Evidence for electronically-driven ferroelectricity in a strongly correlated dimerized bedt-ttf molecular conductor

By applying measurements of the dielectric constants and relative length changes to the dimerized molecular conductor $\ensuremath{\kappa}\text{\ensuremath{-}}(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF}{)}_{2}\mathrm{Hg}(\mathrm{SCN}{)}_{2}\mathrm{Cl}$, we provide evidence for order-disorder type electronic ferroelectricity that is driven by the charge order within the $(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF}{)}_{2}$ dimers and stabilized by a coupling to the anions. According to our density functional theory calculations, this material is characterized by a moderate strength of dimerization. This system thus bridges the gap between strongly dimerized materials, often approximated as dimer-Mott systems at $1/2$ filling, and nondimerized or weakly dimerized systems at $1/4$ filling, exhibiting a charge order. Our results indicate that intradimer charge degrees of freedom are of particular importance in correlated $\ensuremath{\kappa}\text{\ensuremath{-}}(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF}{)}_{2}X$ salts and can create novel states, such as electronically driven multiferroicity or charge-order-induced quasi-one-dimensional spin liquids.