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

Abstract

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.