Charge ordering in (TMTTF)2X salts

We show that the newly discovered phase transitions in (TMTTF)(2)X (X = PF6, AsF6, SbF6, ReO4) are characterised by a huge divergent peak in the low frequency dielectric constant. These phase transitions can be explained by combined effect of charge disproportionation on TMTTF molecules and the uniform shift of anions, yielding thus a macroscopic ferroelectric polarisation. Anion ordering transition, which occurs for non-centrosymmetrical anions, has a specific effect depending on the distortion vector: it destabilises the ferroelectric state in the (TMTTF)(2)ReO4 and brings an antiferroelectric character to the transition in (TMTTF)(2)SCN. (C) 2002 Elsevier Science B.V. All rights reserved.