Structural Aspects of the Bechgaard Salts and Related Compounds

Abstract

Structural aspects of the Bechgaard salts (TMTSF) 2 X and related 2:1 salts, mostly based on the sulfur donors TMTTF and BCPTTF as well as on the hybrid TMDTDSF donor, are considered in relationship with their physical properties. The basic structure of these salts is reviewed with a special emphasize on the dimerization of the organic stack and on the disorder of the anions (X). Then the density waves instabilities driven by the donor stacks are discussed. It is shown in particular that (TMTSF)2PF6 exhibits a mixed 2k F CDW-SDW ground state. The 4k F charge localization phenomena and the 2k F 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 spin Peierls instabilities of (TMTTF)2PF6 and of (BCPTTF)2PF6 are more particularly detailed. General aspects of the anion ordering transitions exhibited by salts with non centrosymmetrical anions are pointed out as well as the influence of such transitions on the electronic properties. Finally the relevance of these structural features is briefly discussed by comparing the Bechgaard salts series' of quasi-one dimensional conductors to the family of two dimensional (BEDTTTF) 2 X conductors.