Structural Aspects of the Bechgaard Salts and Related Compounds

Structural aspects of the Bechgaard salts (TMTSF)2X and related 2:1 salts. mostly based on the sulfur donors TMTTF and BCPOITF as well as on the hybrid TMDTDSF dorlor, are considered in relationship with their physical properties. The basic structure of these softs is revie~ved with a special emphasize on the dimerization of the organic stack and on the disorder of the aurons IX). 0lhen the density waves instabilities driven by the donor stacks are discussed It is shown in particular that (0lM0lSF)2PF6 exhibits a rrixed 2kF CDW-SDW ground state. The 4kF charge localization phenoinena and the 2kF instability of the organic stacks are described in relationship with the structural dimerization and the importance of Coulomb interactions. In the strong charge localization case the spm Peierls mstabilities of (TMTTF)2PF6 and of (BCPTTF)2PF6 are more particularly detailed. General aspects of the amon ordering transitionq exhibited by salts with non centrosymmetrical aurons are pointed Dut as well as the influence of such transitions on the electronic properties. Finally the relevance of these structural features is briefly discussed by corrparing the Bechgaard softs serres' of quasi-one dimensional conductors to the family of t~vo dirrensional (BEDOITTF)2X conductors.