  A detailed investigation of the out-of-plane electrical properties of
charge-ordered alpha-(BEDT-TTF)2I3 provides clear evidence for
ferroelectricity. Similar to multiferroic alpha-(BEDT-TTF)2Cu[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.
