Comparative X-Ray Diffuse-Scattering Studies Of Some Superconducting And Non Superconducting 1-2 Salts (Tmtsf)2x And (Tmttf)2x - With X=Pf6,Asf6,Clo4,No3

X-ray diffuse scattering experiments performed on the highly conducting (TMTSF) 2 X compounds (X=PF 6 , AsF 6 , ClO 4 , NO 3 ) reveal no charge density wave instabilities, whether they display superconducting properties or not. Instead, another type of phase transition, not observed for the superconducting members (PF 6 , AsF 6 , ClO 4 ), takes place in the non-superconducting (TMTSF) 2 NO 3 , and involves an ordering of the counter ions, which doubles the lattice constant in chain direction. Similar experiments performed on the less conducting (TMTTF) 2 X compounds (X = ClO 4 , PF 6 ), which do not present a superconducting state, either reveal the ordering transition of the counter ion (ClO 4 ) or the quasi 1-D 2k F scattering characteristic of a charge density wave instability (PF 6 ). Consequences on the dimensionality of the Fermi surface and on the superconducting instability are discussed.