Structural features of the Bechgaard salts and related compounds

Abstract
We present an overview of the structural instabilities and of the structural transitions exhibited by the Bechgaard salts (TMTSF)₂X, the Fabre salts (TMTTF)₂X, and related compounds. In the (TMTTF)₂X and (BCPTTF)₂X series with centrosymmetric anions X, such as PF₆, a 2k_F bond instability leads to the formation of a spin-Peierls (SP) ground state. A quantitative analysis of its one dimensional (1D) pretransitional fluctuations, previously performed in the BCPTTF series, is extended to the inorganic SP compound CuGeO₃. In the (TMTSF)₂X series, the 1D 2k_F charge density wave (CDW) structural fluctuations vanish at low temperature when the spin density wave correlations develop. However the detection of very weak 2k_F and 4k_F satellite reflections shows that the density wave ground state of (TMTSF)₂PF₆ unexpectedly presents a mixed spin-charge character. In salts with X non-centrosymmetric, the anions, disordered at room temperature, order in a superlattice upon cooling. However, because of their coupling with the organic sublattice, these ordering lead to important modifications of the electronic properties. Finally, we report a quantitative analysis of the 1D 2k_F and 4k_F CDW lattice fluctuations observed in the TTF-TCNQ family, allowing to extract the exponent K_p governing the collective instabilities of the Luttinger liquid.