Charge and anion ordering phase transitions in (TMTTF)₂X salt conductors

We report measurements of the low frequency conductivity and dielectric permittivity of quasi-one-dimensional organic (TMTTF)₂X salts with non-centrosymmetrical anions X = ReO₄ and SCN. We show that the `structureless transition' at 227.5 K in (TMTTF)₂ReO₄ is due to charge ordering and has a ferroelectric character. The anion ordering transition strongly affects the dielectric response: it suppresses the polarizability in (TMTTF)₂ReO₄ and induces probably an antiferroelectric state in (TMTTF)₂SCN.