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)zX series with centrosymmetric anions X, such as PF6, a 2k~ bond instability leads to the formation of a spin-Peierls (SP) ground state. A quantitative analysis of its one dimensional (ID) pretransitional fluctuations, previously performed in the BCPTTF series, is extended to the inorganic SP compound CuGeOs. In the (TMTSF)2X series, the 1D 2k~ charge density wave (CDW) structural fluctuations vanish at low temperature when the spin density wave correlations develop. However the detection of very weak 2k~ and 4k~ 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~ and 4k~ CDW lattice fluctuations observed in the TTF-TCNQ family, allowing to extract the exponent K, governing the collective instabilities of the Luttinger liquid.