Ferroelectric properties of charge-ordered α-(BEDT-TTF)2I3

A detailed investigation of the out-of-plane electrical properties of charge-ordered alpha-(BEDT-TTF)(2)I-3 provides clear evidence for ferroelectricity. Similar to multiferroic kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Cl, the polar order in this material is ascribed to the occurrence of bond- and site-centered charge order. Dielectric response typical for relaxor ferroelectricity is found deep in the charge-ordered state. We suggest an explanation in terms of the existence of polar and nonpolar stacks of the organic molecules in this material, preventing long-range ferroelectricity. The results are discussed in relation to the formation or absence of electronic polar order in related charge-transfer salts.